feat: add async durable session storage

[roackb2]

Summary

• replace synchronous whole-collection session persistence with an async, record-oriented repository contract
• add optimistic revisions, compare-and-swap updates/deletes, stable cursor pagination, and explicit storage errors
• harden the default JSON adapter with immutable revision bodies, atomic catalog replacement, and cross-process writer locking
• await durable run acceptance before the control plane reports an accepted prompt
• document zero-config local JSON usage and PostgreSQL adapter semantics

Verification

• yarn test:unit — 114 files, 679 tests passed
• yarn test:integration — 32 files, 295 tests passed
• yarn typecheck
• yarn build
• yarn lint
• git diff --check

Scope

This PR establishes the session durability foundation. Artifact, archive, trace, and object-storage adapters remain separate tracked follow-up slices.

The concurrent root README refresh is intentionally excluded because another task owns it.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Update session watchers for revisioned storage

With this new <encoded-id>.<revision>.json layout, no new write touches chat-sessions/${sessionId}.json, and later saves move on to .2, .3, etc. I checked ControlPlaneChatSessionsController.subscribeLiveEvents and the REST ChatSessionEventsRestController: both still watch resolveFilePath(..., ${sessionId}.json), so selected-session streams for new sessions only emit waiting and never send session.updated when another client, turn, or settings write persists. The UI can miss durable updates until a run terminal/poll/manual refresh; watch the catalog/directory or publish session update events from the service when revisioned writes complete.

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