v2: Build resumable per-provider local archives

[jmcte]

Problem / intent

The current watch cache serializes four whole command payloads. It is useful polling proof, but it is not an incremental crawler: there are no source cursors, fingerprints, tombstones, migrations, retention rules, or provider freshness contracts.

Acceptance criteria

• Define a versioned archive contract shared by providers while keeping domain schemas separate.
• Persist provider cursor/source fingerprint, last attempt, last success, freshness, item counts, and structured failure state.
• Support idempotent upsert and explicit deletion/tombstone handling.
• Use private local storage with documented permissions, retention, backup, and redacted logging behavior.
• Expose bounded sync and status APIs without embedding JSON payloads inside JSON strings.
• Prove restart/resume, migration, partial failure, and retention with synthetic tests.
• Do not archive high-sensitivity bodies or media unless a later provider issue explicitly opts in.

Dependencies

• Depends on v2: Define a versioned provider capability manifest #89 and v2: Snapshot live SQLite stores consistently before querying #90.
• v2 follow-up to closed issue Add persistent local cache and watch mode for OpenClaw polling #23.