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/summarize_mode.feature

@@ -19,9 +19,12 @@ Feature: Choose how new videos get summarized
     And it is not summarized automatically
     And I can still summarize it on demand with "Summarize"
 
-  Scenario: Manual mode is the default and leaves new videos unsummarized
-    Given I have not changed my summarization mode
-    Then my mode is "manual"
+  Scenario: Automatic is the default for a new user
+    Given I have just registered
+    Then my summarization mode is "auto"
+
+  Scenario: Manual mode leaves new videos unsummarized
+    Given my summarization mode is "manual"
     When a subscribed channel posts a new video with captions
     Then the video appears in my list with no summary
     And nothing is summarized until I request it