feat: add concurrent agents with dependency system and delightful UI

Major feature implementation for parallel agent execution with dependency-aware
scheduling and an engaging multi-agent UI experience.

Backend Changes:
- Add parallel_orchestrator.py for concurrent feature processing
- Add api/dependency_resolver.py with cycle detection (Kahn's algorithm + DFS)
- Add atomic feature_claim_next() with retry limit and exponential backoff
- Fix circular dependency check arguments in 4 locations
- Add AgentTracker class for parsing agent output and emitting updates
- Add browser isolation with --isolated flag for Playwright MCP
- Extend WebSocket protocol with agent_update messages and log attribution
- Add WSAgentUpdateMessage schema with agent states and mascot names
- Fix WSProgressMessage to include in_progress field

New UI Components:
- AgentMissionControl: Dashboard showing active agents with collapsible activity
- AgentCard: Individual agent status with avatar and thought bubble
- AgentAvatar: SVG mascots (Spark, Fizz, Octo, Hoot, Buzz) with animations
- ActivityFeed: Recent activity stream with stable keys (no flickering)
- CelebrationOverlay: Confetti animation with click/Escape dismiss
- DependencyGraph: Interactive node graph visualization with dagre layout
- DependencyBadge: Visual indicator for feature dependencies
- ViewToggle: Switch between Kanban and Graph views
- KeyboardShortcutsHelp: Help overlay accessible via ? key

UI/UX Improvements:
- Celebration queue system to handle rapid success messages
- Accessibility attributes on AgentAvatar (role, aria-label, aria-live)
- Collapsible Recent Activity section with persisted preference
- Agent count display in header
- Keyboard shortcut G to toggle Kanban/Graph view
- Real-time thought bubbles and state animations

Bug Fixes:
- Fix circular dependency validation (swapped source/target arguments)
- Add MAX_CLAIM_RETRIES=10 to prevent stack overflow under contention
- Fix THOUGHT_PATTERNS to match actual [Tool: name] format
- Fix ActivityFeed key prop to prevent re-renders on new items
- Add featureId/agentIndex to log messages for proper attribution

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/services/process_manager.py

@@ -80,6 +80,8 @@ class AgentProcessManager:
         self._output_task: asyncio.Task | None = None
         self.yolo_mode: bool = False  # YOLO mode for rapid prototyping
         self.model: str | None = None  # Model being used
+        self.parallel_mode: bool = False  # Parallel execution mode
+        self.max_concurrency: int | None = None  # Max concurrent agents
 
         # Support multiple callbacks (for multiple WebSocket clients)
         self._output_callbacks: Set[Callable[[str], Awaitable[None]]] = set()
@@ -241,13 +243,21 @@ class AgentProcessManager:
                     self.status = "stopped"
                 self._remove_lock()
 
-    async def start(self, yolo_mode: bool = False, model: str | None = None) -> tuple[bool, str]:
+    async def start(
+        self,
+        yolo_mode: bool = False,
+        model: str | None = None,
+        parallel_mode: bool = False,
+        max_concurrency: int | None = None,
+    ) -> tuple[bool, str]:
         """
         Start the agent as a subprocess.
 
         Args:
             yolo_mode: If True, run in YOLO mode (no browser testing)
             model: Model to use (e.g., claude-opus-4-5-20251101)
+            parallel_mode: If True, run multiple features in parallel
+            max_concurrency: Max concurrent agents (default 3 if parallel enabled)
 
         Returns:
             Tuple of (success, message)
@@ -261,6 +271,8 @@ class AgentProcessManager:
         # Store for status queries
         self.yolo_mode = yolo_mode
         self.model = model
+        self.parallel_mode = parallel_mode
+        self.max_concurrency = max_concurrency
 
         # Build command - pass absolute path to project directory
         cmd = [
@@ -278,6 +290,11 @@ class AgentProcessManager:
         if yolo_mode:
             cmd.append("--yolo")
 
+        # Add --parallel flag if parallel mode is enabled
+        if parallel_mode:
+            cmd.append("--parallel")
+            cmd.append(str(max_concurrency or 3))  # Default to 3 concurrent agents
+
         try:
             # Start subprocess with piped stdout/stderr
             # Use project_dir as cwd so Claude SDK sandbox allows access to project files
@@ -340,6 +357,8 @@ class AgentProcessManager:
             self.started_at = None
             self.yolo_mode = False  # Reset YOLO mode
             self.model = None  # Reset model
+            self.parallel_mode = False  # Reset parallel mode
+            self.max_concurrency = None  # Reset concurrency
 
             return True, "Agent stopped"
         except Exception as e:
@@ -422,6 +441,8 @@ class AgentProcessManager:
             "started_at": self.started_at.isoformat() if self.started_at else None,
             "yolo_mode": self.yolo_mode,
             "model": self.model,
+            "parallel_mode": self.parallel_mode,
+            "max_concurrency": self.max_concurrency,
         }