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/websocket.py

@@ -78,6 +78,9 @@ ORCHESTRATOR_PATTERNS = {
     'testing_complete': re.compile(r'Feature #(\d+) testing (completed|failed)'),
     'all_complete': re.compile(r'All features complete'),
     'blocked_features': re.compile(r'(\d+) blocked by dependencies'),
+    'drain_start': re.compile(r'Graceful pause requested'),
+    'drain_complete': re.compile(r'All agents drained'),
+    'drain_resume': re.compile(r'Resuming from graceful pause'),
 }
 
 
@@ -562,6 +565,30 @@ class OrchestratorTracker:
                     'All features complete!'
                 )
 
+            # Graceful pause (drain mode) events
+            elif ORCHESTRATOR_PATTERNS['drain_start'].search(line):
+                self.state = 'draining'
+                update = self._create_update(
+                    'drain_start',
+                    'Draining active agents...'
+                )
+
+            elif ORCHESTRATOR_PATTERNS['drain_complete'].search(line):
+                self.state = 'paused'
+                self.coding_agents = 0
+                self.testing_agents = 0
+                update = self._create_update(
+                    'drain_complete',
+                    'All agents drained. Paused.'
+                )
+
+            elif ORCHESTRATOR_PATTERNS['drain_resume'].search(line):
+                self.state = 'scheduling'
+                update = self._create_update(
+                    'drain_resume',
+                    'Resuming feature scheduling'
+                )
+
             return update
 
     def _create_update(