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>

ui/src/components/KanbanColumn.tsx

@@ -1,11 +1,13 @@
 import { FeatureCard } from './FeatureCard'
 import { Plus, Sparkles } from 'lucide-react'
-import type { Feature } from '../lib/types'
+import type { Feature, ActiveAgent } from '../lib/types'
 
 interface KanbanColumnProps {
   title: string
   count: number
   features: Feature[]
+  allFeatures?: Feature[]  // For dependency status calculation
+  activeAgents?: ActiveAgent[]  // Active agents for showing which agent is working on a feature
   color: 'pending' | 'progress' | 'done'
   onFeatureClick: (feature: Feature) => void
   onAddFeature?: () => void
@@ -23,12 +25,18 @@ export function KanbanColumn({
   title,
   count,
   features,
+  allFeatures = [],
+  activeAgents = [],
   color,
   onFeatureClick,
   onAddFeature,
   onExpandProject,
   showExpandButton,
 }: KanbanColumnProps) {
+  // Create a map of feature ID to active agent for quick lookup
+  const agentByFeatureId = new Map(
+    activeAgents.map(agent => [agent.featureId, agent])
+  )
   return (
     <div
       className="neo-card overflow-hidden"
@@ -86,6 +94,8 @@ export function KanbanColumn({
                 feature={feature}
                 onClick={() => onFeatureClick(feature)}
                 isInProgress={color === 'progress'}
+                allFeatures={allFeatures}
+                activeAgent={agentByFeatureId.get(feature.id)}
               />
             </div>
           ))