Migrate the conversation summary App Server methods to ThreadStore
Because this app server api allows explicitly fetching the thread by
rollout path, intercept that case in the app server code and (a) route
directly to underlying local thread store methods if we're using a local
thread store, or (b) throw an unsupported error if we're using a remote
thread store. This keeps the thread store API clean and all filesystem
operations inside of the local thread store, which pushing the
"fundamental incompatibility" check as early as possible.
- Migrates unloaded `thread/name/set` and `thread/memoryModeSet`
app-server writes behind the generic
`ThreadStore::update_thread_metadata` API rather than adding one-off
store methods for setting thread name or memory mode.
- Implements the local ThreadStore metadata patch path for thread name
and memory mode, including rollout append, legacy name index updates,
SessionMeta validation/update, SQLite reconciliation, and re-reading the
stored thread.
- Adds focused local thread-store unit coverage plus app-server
integration coverage for the migrated unloaded write paths.
Introduce a ThreadStore interface for mediating access to the filesystem
(rollout jsonl files + sqlite db) based thread storage.
In later PRs we'll move the existing fs code behind a "local"
implementation of this ThreadStore interface.
This PR should be a no-op behaviorally, it only introduces the
interface.
Tom