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>

docs/use-cases/connect_account.feature

@@ -10,6 +10,8 @@ Feature: Connect and manage video accounts
     And my refresh token is stored only as a secret reference
     And my subscriptions are synced
 
+  @pending
+  # Vimeo connect is not built yet (provider label exists; no connect flow or test).
   Scenario: Connect a Vimeo account
     Given I have no connected video accounts
     When I connect my Vimeo account
@@ -28,6 +30,9 @@ Feature: Connect and manage video accounts
     And no new videos are watched for that connection
     And my existing summaries remain readable
 
+  @pending
+  # Per-provider BYO credential config is not built as a web flow yet (the summarizer
+  # supports a fallback endpoint, but there is no user-facing BYO setup + its test).
   Scenario Outline: BYO AI credential is optional and per-provider
     When I configure a BYO provider "<provider>"
     Then the credential is stored only as a secret reference