add database layer for subagent job queue

Phase 1 of subagent architecture redesign. Adds Jobs.hs with:
- subagent_jobs table for persistent job queue
- subagent_events table for lifecycle event log
- Job CRUD: create, claim, complete, fail, cancel
- Event logging for started, progress, tool_call, error, completed
- Queries: pending jobs, running jobs, active summary for context
- All tests passing

This enables:
- Jobs survive process restarts
- Workers can claim and execute jobs independently
- Events flow back to Ava's context for collaborative debugging

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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