sessions: design + implement thread metadata / archive endpoints

[hrhrng]

Background

`apps/main/src/routes/sessions.ts:1618` and `:1634` return 501:

• `GET /v1/sessions/:id/threads/:thread_id` — thread metadata
• `POST /v1/sessions/:id/threads/:thread_id/archive` — thread archive

AMA's threads are fork-out / branch-out sub-conversations within a session. OMA's SessionDO is single-threaded — there's no native concept of a thread distinct from the session itself.

Work

This isn't fillable by alignment alone; it needs a product decision on how OMA models multi-thread conversations:

1. Fold OMA's "single thread per session" into the AMA shape — every session has exactly one thread (synthetic id), endpoints return that
2. Actually implement multi-thread (would need SessionDO to track multiple parallel histories — significant work)
3. Document as not-supported, leave 501

Why deferred from #PR

Product decision, not a wire alignment gap.

References

`apps/main/src/routes/sessions.ts:1611-1640`