feat: DB-backed agent session/run store with claim+lock lifecycle

[foo-bender]

Summary

• add clawpress_agent_sessions + clawpress_agent_runs custom table stores with dbDelta migrations
• wire activation to create both runtime tables
• implement run claim/lock lifecycle with CAS-style guards, stale-lock reclaim, and completion transitions
• update parent session run metadata/failure counters on completion
• add unit coverage for claim success, claim collision, stale reclaim, and completion transitions

Validation

• composer lint
• composer test

Closes #13