Add inactive session artifact janitor

[zzj3720]

Summary

• add a session artifact janitor that cleans rebuildable macOS worktree artifacts for inactive sessions
• gate cleanup on no active turn, no pending inbound work, and no running background jobs
• document and test the new cleanup knobs

Testing

• pnpm exec vitest run test/config.test.ts test/session-artifact-janitor.test.ts
• pnpm build
• manual smoke: temp-dir session manager run showing inactive .build removed while active session .build remained