agentd daemon: Replace Process.createProcess with systemd delegation

Refactor Daemon.hs to stop managing Pi processes directly and delegate to systemd instead. This is the core daemon refactor.

1. Remove RunningAgent and in-memory process tracking:

• Delete data RunningAgent (has raStdinHandle, raWorktreePath, raWorktreeRepoRoot)
• Delete dsRunning :: TVar (Map Text RunningAgent) from DaemonState
• Delete monitorPiAgent function
• Delete readPiOutput function
• Delete readPiStderr function
• Delete sendToPiProcess (replace with FIFO write)

2. Remove git worktree code:

• Delete prepareWorkspace function (creates git worktrees)
• Delete cleanupWorktree function
• Delete the DELETE /agents/:id/worktree API endpoint
• Remove raWorktreePath and raWorktreeRepoRoot from all code paths

3. Update POST /agents (spawn):

• Instead of Process.createProcess, call the same logic as agentd agent create + agentd agent start
• SpawnRequest includes: name (optional, auto-generate if missing), provider, model, cwd (required), thinking level
• No automatic worktree creation

4. Update POST /agents/:id/send:

• Write to FIFO at ~/.local/state/pi-agents/<name>.fifo instead of raStdinHandle
• Record message in DB

5. Update GET /agents/:id:

• Read from systemd state + DB, not in-memory TVar
• Call systemctl --user is-active pi-agent@<name> and merge with DB record

The daemon becomes stateless w.r.t. processes. All process state lives in systemd. All persistent state lives in SQLite.

Files to modify:

• Omni/Agentd/Daemon.hs (major refactor, ~1888 lines currently)