Add an agent-oriented overview command

[laotoutou]

Summary

• add sandnote overview as a notebook-level overview surface
• expose a stable JSON contract plus a denser default agent-readable text view
• show explicit resume state, thread-entry-artifact relations, and no-live-thread cases
• update smoke coverage, README, and the sandnote skill to prefer default overview for agents

Closes #49

Testing

• go test ./...
• go test ./internal/cli/...
• bash scripts/smoke.sh