feat: mechanical date filtering for collected data (vana context --since)

[tnunamak]

Context

Connected data now persists per-source at ~/.dataconnect/results/{source}.json. To support the vana next / vana context feature (generate agent prompts from personal data), we need the ability to extract "what changed in the last 24 hours" mechanically.

Approaches

Diff between runs: Store previous collection results and diff with current. New/changed items = the delta. Requires result versioning (multiple snapshots per source).

Connector-level timestamp annotation: Add metadata to connector schemas declaring which fields are timestamps and sort order. Then vana context --since 24h can filter mechanically.

Upstream support: Some platforms expose activity feeds or change logs. Connectors could extract these directly instead of diffing full snapshots.

Current workaround

A skill can teach the agent to parse timestamps from the raw data (most connectors include created_at, updated_at, or similar fields). This works for testing the vana next experience without mechanical filtering.

Dependencies

• Per-source result storage (done: 32efd1c)
• Result versioning / snapshot history (not yet built)
• Connector metadata schema annotations (not yet standardized)