feat(daemon): arc migrate v2-to-v3 CLI — ingest v2 state into SQLite

[Codename-11]

Summary

• Adds arc migrate v2-to-v3 CLI command (offline, does not require the daemon) that ingests v2 state into the v3 SQLite store at ~/.arc/arc.db.
• New migrateV2ToV3() function in packages/daemon/src/migrations/v2-to-v3.ts — reads history.json, per-profile chat-sessions/*.json, and activity.log (or logs/events.ndjson); writes rows into agents, agent_events, and meta.
• Idempotence is guarded by a meta row (migration_v2_to_v3_completed_at). A second invocation reports "already migrated, nothing to do".
• Non-destructive: v2 files are preserved, never deleted or modified.
• Supports --dry-run (counts only, no DB writes) and --json.
• Fixture-based vitest covering: ingestion, idempotence, dry-run, non-destructiveness, and missing-history graceful path.

Mapping

• Each history.json launch entry becomes one agents row (status=completed, launch_mode=native, deterministic v2-<sha1> id, metadata JSON preserves tool/outcome/exitCode).
• Chat session messages land as agent_events with epoch=1: assistant/user/system → stdout, toolCalls[] → tool_call (+ tool_result if result/error present), role=tool → tool_result.
• Activity log lines land as agent_events with kind=status, correlated to an agent via timestamp window (+/- 10 min, same profile). Unmatched lines are counted as skipped.

Test plan

• npx tsc --noEmit passes
• npx vitest run tests/migrate-v2-to-v3.test.ts — 5/5 pass
• CLI smoke (dry-run emits JSON counts, no db written; real run creates arc.db; second run reports "already migrated")
• Reviewer spot-check DB schema and event kinds