feat: add graceful pause (drain mode) for running agents

[CaitlynByrne]

Summary

• File-based signal (.pause_drain) lets the orchestrator finish current work before pausing instead of hard-freezing the process tree
• New status states pausing / paused_graceful flow through WebSocket to the UI
• UI adds a Pause button, draining indicator, and Resume button

Changes

• parallel_orchestrator.py — detect .pause_drain signal file, drain gracefully
• server/services/process_manager.py — new pause/resume endpoints and status management
• server/routers/agent.py — REST API for pause/resume
• autoforge_paths.py — path constant for .pause_drain signal file
• UI: AgentControl.tsx — pause button with draining state, OrchestratorStatusCard.tsx — status display
• WebSocket and type definitions updated for new statuses

Test plan

• Start a parallel agent run and click Pause — verify agents finish current feature before stopping
• Confirm UI shows "draining" indicator while agents wind down
• Click Resume and verify agents pick back up
• Verify .pause_drain file is created on pause and removed on resume
• Test that hard stop still works while in paused/draining state

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