CompletionCache: zero test coverage and uses Instant.now() instead of Clock.instant

[Sootopolis]

CompletionCache has no test suite at all. Untested behaviours that are easy to break:

• TTL boundary in clubsStale (CompletionCache.scala:37-44)
• seedClubs no-op-when-file-exists plus the deliberate epoch-0 mtime stamp (:68-77)
• writeClubs truncate semantics and the empty-list trailing-newline branch (:57-61)
• appendJob dedup + MaxRecentJobs cap (:80-87)

Separately, CompletionCache.scala:41 uses Instant.now() rather than Clock.instant, so it bypasses ZIO's TestClock — the TTL boundary can't be tested deterministically as written. Switch it, per the repo's determinism stance (#65).

Also worth adding while nearby:

• a stale / unmanaged current_club case in TestClubResolver
• a TestRoutes case for an existing-but-unmanaged club (see Gate job submission on managed status (JobRoutes.submitClubJob) #177)
• a TestConfigWriter case for clearing current_club (see 'club remove' silently leaves a stale current_club, and jobs still run against the removed club #174)

[Sootopolis]

Shipped in #183, squash-merged as a318e26.