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/landing_page.feature

@@ -19,6 +19,9 @@ Feature: Public landing page
     Then I see a link to my summaries
     And I see a way to log out
 
+  @pending
+  # Behaviour ships (logout redirects to /welcome) but is not unit-tested: logout lives in
+  # the OIDC Auth impl and StubAuth has no routes to exercise it cheaply.
   Scenario: Logging out returns to the welcome page
     Given I am logged in
     When I log out