### Change summary
Defer rollout file creation until needed.
* Add a core API to force rollout persistence for loaded non-ephemeral threads:
* seeds initial context if needed
* flushes rollout and returns persisted path
Add concurrency guard to make lazy rollout initialization idempotent under concurrent calls.
Add centralized app-server rollout-path resolver that:
* uses in-memory thread state when loaded
* forces persistence on demand for rollout-dependent calls
* falls back to on-disk lookup for unloaded threads
* maps ephemeral threads to invalid-request errors for rollout-dependent operations
Route rollout-dependent endpoints through the resolver (v2 + shared legacy surfaces), including:
* thread/archive
* thread/resume (thread-id path)
* thread/fork (thread-id path)
* resumeConversation
* forkConversation
* thread summary by thread id
* detached review parent-thread path resolution
* feedback include_logs rollout resolution
Remove stale cached rollout-path assumptions in rollback/detached-review flows by resolving via thread id when needed.
No wire-schema changes; behavior-only change.
v1 compatibility is not expanded in this PR.
### Tests updated/added
* thread_start: assert rollout is absent immediately after thread/start; created after first completed turn.
* thread_resume: resume by thread id succeeds for just-started thread via on-demand persistence; path-vs-thread-id precedence test updated.
* thread_fork: fork by thread id succeeds for just-started thread.
* thread_archive: archive succeeds for just-started thread and materializes before archive.
* thread_unarchive: adjusted for deferred creation timing.
* thread_rollback: rollback path no longer depends on stale cached rollout path.
* Detached review targeted test verified for lazy path behavior.
* Core tests for new persistence API
## Summary
- Touch restored rollout files on `thread/unarchive` so `updatedAt`
reflects the unarchive time.
- Add a regression test to ensure unarchiving bumps `updated_at` from an
old mtime.
## Notes
This fixes the UX issue where unarchived old threads don’t reappear near
the top of recent threads.
## Summary
- Adds a new `thread/unarchive` RPC to move archived thread rollouts
back into the active `sessions/` tree.
## What changed
- **Protocol**
- Adds `thread/unarchive` request/response types and wiring.
- **Server**
- Implements `thread_unarchive` in the app server.
- Validates the archived rollout path and thread ID.
- Restores the rollout to `sessions/YYYY/MM/DD/...` based on the rollout
filename timestamp.
- **Core**
- Adds `find_archived_thread_path_by_id_str` helper for archived
rollouts.
- **Docs**
- Documents the new RPC and usage example.
- **Tests**
- Adds an end-to-end server test that:
1) starts a thread,
2) archives it,
3) unarchives it,
4) asserts the file is restored to `sessions/`.
## How to use
```json
{ "method": "thread/unarchive", "id": 24, "params": { "threadId": "<thread-id>" } }
```
## Author Codex Session
`codex resume 019bf158-54b6-7960-a696-9d85df7e1bc1` (soon I'll make this
kind of session UUID forkable by anyone with the right
`session_object_storage_url` line in their config, but for now just
pasting it here for my reference)
