feat: add async conversation archive repository

[roackb2]

Summary

• add an async, host-injected ChatArchiveRepository for compacted raw transcripts, rolling summaries, and manifests
• make compaction save through one durable append boundary before session state can reference the archive
• upgrade the default file adapter to async I/O, serialized appends, atomic manifest replacement, strict corruption handling, and path-safe locators
• propagate the repository through the engine, quickstart runner, turn preflight/persistence, direct shell, and control-plane compaction
• export strict adapter-authoring codecs/errors and document a PostgreSQL/Drizzle-friendly storage shape

Why

A remote ChatSessionRepository could reopen active session state on another replica, but compacted history still referenced local archive files. This gives hosts a separate archive persistence port while preserving the zero-configuration local layout and keeping SQL, identity, tenancy, and object-store choices outside Heddle.

Reliability semantics

• summarization completes before archive storage is mutated
• append makes raw messages, summary text, and the returned manifest visible as one repository operation
• repository failures reject the turn/manual compaction and restore any temporary running metadata; exact active history is retained
• valid v1 local manifests remain readable, while malformed, mismatched, missing-summary, and escaping-locator state fails loudly
• path and summaryPath stay backward-compatible field names but are now documented as repository-owned opaque locators

Verification

• yarn lint
• yarn typecheck
• yarn test — 703 unit and 310 integration tests passed
• yarn build

The build only reports the existing Vite chunk-size warnings.

Deliberate boundaries

• no database/ORM/object-store dependency in Heddle
• no model-visible remote raw-archive reader in this slice
• traces, memory, and active-run/SSE coordination remain outside this storage port