test(bdd): add scenario name-coverage gate (no godog)

Close the gap where docs/use-cases/*.feature claimed to be the behavior spec
but nothing executed them — so scenarios drifted (the stale "auto summarizes
every new video" and "manual is the default" were proof).

Decision (per issue #5 BDD-runner fork): no godog — keep .feature as design
records, add a cheap name-coverage gate instead. TestScenarioCoverage parses
every scenario and asserts each non-@pending one maps to an existing Go test in
the scenarioCoverage manifest; it flags unmapped scenarios, missing/renamed
tests, and stale entries. It checks the link, not that the test exercises the
scenario (the deliberate trade for skipping godog).

Also:
- Fix the stale ADR-018 drift: "Manual is the default" -> auto is the default
  for new users; added an explicit default scenario + a plain manual scenario.
- Tag 4 documented-but-unbuilt/untested scenarios @pending with reasons (Vimeo
  connect, BYO config flow, logout->welcome, re-register-after-delete) so they
  are tracked without a false coverage claim.
- CLAUDE.md BDD section now describes the real setup (design records + the gate
  + @pending convention) instead of claiming an executable spec.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/acceptance/scenario_coverage_test.go

@@ -0,0 +1,211 @@
+package acceptance
+
+// This is the name-coverage gate for the BDD spec (see docs/use-cases/*.feature).
+// There is no godog runner — the .feature files are design records, and the real
+// behaviour is covered by the hand-written Go tests across the module. This test
+// keeps the two from drifting in the cheapest honest way: every non-@pending
+// Scenario must have an entry in scenarioCoverage pointing at a Go test that
+// actually exists. It does NOT prove the test exercises the scenario (only godog
+// could); it catches the common drift — "added a scenario, forgot the test", a
+// renamed/deleted covering test, or a scenario removed without cleaning the map.
+//
+// When you add a Scenario: either map it here to its covering test, or tag it
+// @pending in the .feature with a one-line reason for why it has no test yet.
+
+import (
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+// scenarioCoverage maps each non-@pending Scenario name to the Go test that
+// covers it. Keep it in sync with docs/use-cases/*.feature — the test below
+// fails if a scenario is unmapped, a mapped test is missing, or an entry no
+// longer matches a real non-pending scenario.
+var scenarioCoverage = map[string]string{
+	// ai_routing.feature
+	"Local AI produces the summary":                             "TestSummarize_LocalSucceeds",
+	"Local AI fails and the user has a BYO provider configured": "TestSummarize_FallsBackToBYO",
+	"Local AI fails and the user has no BYO provider":           "TestSummarize_LocalFailsNoBYO_NoExternalSend",
+	"A user without BYO never has content sent externally":      "TestSummarize_NoBYO_ContentOnlyLocal",
+
+	// landing_page.feature
+	"An unauthenticated visit to the root is sent to the welcome page":    "TestUnauthenticatedRootRedirectsToWelcome",
+	"The welcome page invites an unauthenticated visitor to start":        "TestWelcomeLoggedOut",
+	"An authenticated user on the welcome page sees their way in and out": "TestWelcomeLoggedIn",
+
+	// connect_account.feature
+	"Connect a YouTube account":                              "TestCallbackExchangesAndRecordsConnection",
+	"Tokens are never stored in the clear":                   "TestCallbackExchangesAndRecordsConnection",
+	"Revoking a connection stops watching but keeps history": "TestDisconnectRemovesTokenAndConnectionKeepsAccount",
+
+	// summarize_mode.feature
+	"Auto mode summarizes recent new videos automatically":           "TestRunOnce_AutoMode_SkipsOldVideos",
+	"Auto mode lists older videos without summarizing them":          "TestRunOnce_AutoMode_SkipsOldVideos",
+	"Automatic is the default for a new user":                        "TestRegisteredUserDefaultsAutoSummarizeOn",
+	"Manual mode leaves new videos unsummarized":                     "TestRunOnce_ManualMode_SkipsUnrequested",
+	"Requesting a summary in manual mode queues it for the next run": "TestRunOnce_ManualMode_ProcessesRequested",
+
+	// registration.feature
+	"A new Dex subject is routed to registration":                               "TestUnregisteredSubjectRedirectedToRegister",
+	"Registering creates the account and its identity mapping":                  "TestRegisterCreatesExactlyOneUserAndIdentity",
+	"A returning subject passes straight through":                               "TestRegisteredSubjectPassesThrough",
+	"Deleting an account removes only my data and leaves other users untouched": "TestDeleteAccountWipesDataAndSecretsAndLogsOut",
+
+	// summarize_new_video.feature
+	"A subscribed channel posts a video that has captions":         "TestSubscribedVideoWithCaptionsIsSummarizedAndDelivered",
+	"A subscribed channel posts a video with no usable transcript": "TestVideoWithNoTranscriptIsSkipped",
+	"A channel I am not subscribed to posts a video":               "TestUnsubscribedChannelVideoIsNotProcessed",
+	"The same video is not summarized twice":                       "TestAlreadySummarizedVideoIsNotReprocessed",
+}
+
+var (
+	scenarioRe = regexp.MustCompile(`^\s*Scenario(?: Outline)?:\s*(.+?)\s*$`)
+	testFuncRe = regexp.MustCompile(`^func (Test\w+)\(`)
+)
+
+// scenario is one parsed Gherkin scenario and whether it is @pending.
+type scenario struct {
+	name    string
+	pending bool
+}
+
+func TestScenarioCoverage(t *testing.T) {
+	root := moduleRoot(t)
+
+	scenarios := parseScenarios(t, filepath.Join(root, "docs", "use-cases"))
+	if len(scenarios) == 0 {
+		t.Fatal("no scenarios parsed from docs/use-cases — wrong path?")
+	}
+	tests := allTestFuncNames(t, root)
+
+	// Index scenario names for the reverse (stale-entry) check.
+	active := map[string]bool{} // non-pending scenario names
+	var pending []string
+	for _, s := range scenarios {
+		if s.pending {
+			pending = append(pending, s.name)
+			continue
+		}
+		active[s.name] = true
+
+		// 1. Every non-pending scenario must be mapped.
+		fn, ok := scenarioCoverage[s.name]
+		if !ok {
+			t.Errorf("scenario %q has no coverage entry — map it in scenarioCoverage to a covering test, or tag it @pending in the .feature", s.name)
+			continue
+		}
+		// 2. The mapped test must actually exist.
+		if !tests[fn] {
+			t.Errorf("scenario %q maps to %q, which is not a Test function anywhere in the module", s.name, fn)
+		}
+	}
+
+	// 3. No stale entries: every map key must be a real, non-pending scenario.
+	for name := range scenarioCoverage {
+		if !active[name] {
+			t.Errorf("scenarioCoverage has entry %q, which is not a current non-pending scenario (renamed, removed, or now @pending?)", name)
+		}
+	}
+
+	if len(pending) > 0 {
+		t.Logf("%d @pending scenario(s) without a test (tracked, not required): %s",
+			len(pending), strings.Join(pending, "; "))
+	}
+}
+
+// parseScenarios reads every *.feature under dir and returns its scenarios with
+// their @pending status. A scenario is @pending when a `@pending` tag line
+// precedes it (tags survive intervening comment lines, the layout these files
+// use); the flag is consumed at the Scenario line and reset afterward.
+func parseScenarios(t *testing.T, dir string) []scenario {
+	t.Helper()
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatalf("read use-cases dir: %v", err)
+	}
+	var out []scenario
+	for _, e := range entries {
+		if e.IsDir() || !strings.HasSuffix(e.Name(), ".feature") {
+			continue
+		}
+		b, err := os.ReadFile(filepath.Join(dir, e.Name()))
+		if err != nil {
+			t.Fatalf("read %s: %v", e.Name(), err)
+		}
+		pending := false
+		for _, line := range strings.Split(string(b), "\n") {
+			trimmed := strings.TrimSpace(line)
+			if strings.HasPrefix(trimmed, "@") {
+				if strings.Contains(trimmed, "@pending") {
+					pending = true
+				}
+				continue
+			}
+			if m := scenarioRe.FindStringSubmatch(line); m != nil {
+				out = append(out, scenario{name: m[1], pending: pending})
+				pending = false
+			}
+			// comment (#) and step lines leave a set @pending intact until the
+			// scenario consumes it; a blank line between scenarios is harmless.
+		}
+	}
+	return out
+}
+
+// allTestFuncNames walks the module for `func TestXxx(` declarations, excluding
+// this file (whose regex literal would otherwise look like a definition).
+func allTestFuncNames(t *testing.T, root string) map[string]bool {
+	t.Helper()
+	self := "scenario_coverage_test.go"
+	names := map[string]bool{}
+	err := filepath.WalkDir(root, func(path string, d os.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() {
+			if d.Name() == ".git" {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+		if !strings.HasSuffix(path, "_test.go") || filepath.Base(path) == self {
+			return nil
+		}
+		b, err := os.ReadFile(path)
+		if err != nil {
+			return err
+		}
+		for _, line := range strings.Split(string(b), "\n") {
+			if m := testFuncRe.FindStringSubmatch(line); m != nil {
+				names[m[1]] = true
+			}
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatalf("walk module: %v", err)
+	}
+	return names
+}
+
+// moduleRoot walks up from the working directory to the dir containing go.mod.
+func moduleRoot(t *testing.T) string {
+	t.Helper()
+	dir, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("getwd: %v", err)
+	}
+	for {
+		if _, err := os.Stat(filepath.Join(dir, "go.mod")); err == nil {
+			return dir
+		}
+		parent := filepath.Dir(dir)
+		if parent == dir {
+			t.Fatal("go.mod not found walking up from cwd")
+		}
+		dir = parent
+	}
+}