docs(agent-skill): say what the setting UI test pins, not that it proves the deleted-key shape

The migration's file comment said test/agent-skill-setting-ui.test.ts proves
both shapes. It pins the shipped one (a stored true wins on a device holding the
old false); the deleted-key shape was measured with the same harness and is not
kept as a test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/web/agent-skill-default-migration.ts

@@ -14,7 +14,8 @@
  * page. Every browser that saved App Settings keeps the form in its localStorage blob, and
  * `loadAppSettingsFromServer()` only overwrites a synced key the server HAS: a key the server
  * lacks keeps the device's `false`, the switch loads off, and the next Save of any setting writes
- * `false` back (test/agent-skill-setting-ui.test.ts proves both shapes). A stored `true` wins on
+ * `false` back (measured with the real page code in the JSDOM harness of
+ * test/agent-skill-setting-ui.test.ts, which pins the shipped shape). A stored `true` wins on
  * every device, including a browser still running a cached settings-ui.js from before the upgrade.
  * Every reader treats `true` and an absent key alike (`!== false`).
  *