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

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
where a Pause button, draining indicator, and Resume button are shown.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/routers/agent.py

@@ -175,3 +175,31 @@ async def resume_agent(project_name: str):
         status=manager.status,
         message=message,
     )
+
+
+@router.post("/graceful-pause", response_model=AgentActionResponse)
+async def graceful_pause_agent(project_name: str):
+    """Request a graceful pause (drain mode) - finish current work then pause."""
+    manager = get_project_manager(project_name)
+
+    success, message = await manager.graceful_pause()
+
+    return AgentActionResponse(
+        success=success,
+        status=manager.status,
+        message=message,
+    )
+
+
+@router.post("/graceful-resume", response_model=AgentActionResponse)
+async def graceful_resume_agent(project_name: str):
+    """Resume from a graceful pause."""
+    manager = get_project_manager(project_name)
+
+    success, message = await manager.graceful_resume()
+
+    return AgentActionResponse(
+        success=success,
+        status=manager.status,
+        message=message,
+    )