agentd: Prune old agents and delete pi-agent script

Clean up legacy artifacts from the old agentd process management system.

1. Prune all 26 old agents from agentd SQLite DB (/var/lib/omni/agentd.db):

• fund, test-sse-v2, test-sse-debug, test-canonical, test-model, test-msgs, test-hello
• All pi-* auto-generated agents

All are stopped/idle/failed/pending. None should be preserved.

2. Stop the fund agent if still running: curl -X POST http://127.0.0.1:8400/agents/fund/stop

3. Clean up leftover worktree directories: rm -rf ~/omni/live/_/agentd/worktrees/

4. Delete ~/.local/bin/pi-agent (bash script, replaced by agentd agent commands)

5. Clean up any shell config that references pi-agent

This should be done LAST, after all other unification tasks are complete and verified working.

Files to delete:

• ~/.local/bin/pi-agent
• ~/omni/live/_/agentd/worktrees/ (directory)
• Old records in /var/lib/omni/agentd.db