Preserve the in-flight #359 crash-loop fix recovered from an orphaned agent
worktree (host crashed before it committed). Widens the fail-closed
LinkageError handling to the keyed opens that previously escaped it
(AccountDataMigrator, deferred Room open, headless workers/IdleService via a
new EncryptedCacheWorkerGuard), plus unit + instrumented regression tests.
Not yet validated end-to-end; gate + E2E run to follow.