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/routers/agent.py

@@ -85,6 +85,8 @@ async def get_agent_status(project_name: str):
         started_at=manager.started_at,
         yolo_mode=manager.yolo_mode,
         model=manager.model,
+        parallel_mode=manager.parallel_mode,
+        max_concurrency=manager.max_concurrency,
     )
 
 
@@ -100,8 +102,15 @@ async def start_agent(
     default_yolo, default_model = _get_settings_defaults()
     yolo_mode = request.yolo_mode if request.yolo_mode is not None else default_yolo
     model = request.model if request.model else default_model
+    parallel_mode = request.parallel_mode or False
+    max_concurrency = request.max_concurrency
 
-    success, message = await manager.start(yolo_mode=yolo_mode, model=model)
+    success, message = await manager.start(
+        yolo_mode=yolo_mode,
+        model=model,
+        parallel_mode=parallel_mode,
+        max_concurrency=max_concurrency,
+    )
 
     return AgentActionResponse(
         success=success,