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>

CLAUDE.md

@@ -60,8 +60,14 @@ These caused real mistakes that were caught and corrected; the corrections are l
   stores, and sinks are adapters. Adding a video provider or a sink = a new adapter implementing
   the interface, nothing in the engine changes. This is what keeps "standalone vs homelab" a
   wiring choice (ADR-003).
-- **BDD.** The `docs/use-cases/*.feature` files are the behavior spec. New behavior gets a
-  scenario; the use-case core is tested through fake adapters, not live YouTube/brain.
+- **BDD.** The `docs/use-cases/*.feature` files are the behavior spec (design records — there is
+  no godog runner). New behavior gets a scenario; the use-case core is tested through fake
+  adapters, not live YouTube/brain. A name-coverage gate (`test/acceptance/scenario_coverage_test.go`,
+  `TestScenarioCoverage`) keeps the two from drifting: every non-`@pending` scenario must be
+  mapped to an existing Go test in `scenarioCoverage`. When you add a scenario, either map it to
+  its covering test or tag it `@pending` in the `.feature` with a one-line reason. It checks the
+  *link*, not that the test exercises the scenario — that's the deliberate trade for not running
+  godog (see issue #5 / the BDD-runner decision).
 
 ## Skills (engineering discipline)