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/CelebrationOverlay.tsx

@@ -0,0 +1,120 @@
+import { useCallback, useEffect, useState } from 'react'
+import { Sparkles, PartyPopper } from 'lucide-react'
+import { AgentAvatar } from './AgentAvatar'
+import type { AgentMascot } from '../lib/types'
+
+interface CelebrationOverlayProps {
+  agentName: AgentMascot
+  featureName: string
+  onComplete?: () => void
+}
+
+// Generate random confetti particles
+function generateConfetti(count: number) {
+  return Array.from({ length: count }, (_, i) => ({
+    id: i,
+    x: Math.random() * 100,
+    delay: Math.random() * 0.5,
+    duration: 1 + Math.random() * 1,
+    color: ['#ff006e', '#ffd60a', '#70e000', '#00b4d8', '#8338ec'][Math.floor(Math.random() * 5)],
+    rotation: Math.random() * 360,
+  }))
+}
+
+export function CelebrationOverlay({ agentName, featureName, onComplete }: CelebrationOverlayProps) {
+  const [isVisible, setIsVisible] = useState(true)
+  const [confetti] = useState(() => generateConfetti(30))
+
+  const dismiss = useCallback(() => {
+    setIsVisible(false)
+    setTimeout(() => onComplete?.(), 300) // Wait for fade animation
+  }, [onComplete])
+
+  useEffect(() => {
+    // Auto-dismiss after 3 seconds
+    const timer = setTimeout(dismiss, 3000)
+
+    // Escape key to dismiss early
+    const handleKeyDown = (e: KeyboardEvent) => {
+      if (e.key === 'Escape') {
+        dismiss()
+      }
+    }
+
+    window.addEventListener('keydown', handleKeyDown)
+    return () => {
+      clearTimeout(timer)
+      window.removeEventListener('keydown', handleKeyDown)
+    }
+  }, [dismiss])
+
+  if (!isVisible) {
+    return null
+  }
+
+  return (
+    <div
+      className={`
+        fixed inset-0 z-50 flex items-center justify-center
+        pointer-events-none
+        transition-opacity duration-300
+        ${isVisible ? 'opacity-100' : 'opacity-0'}
+      `}
+    >
+      {/* Confetti particles */}
+      <div className="absolute inset-0 overflow-hidden">
+        {confetti.map((particle) => (
+          <div
+            key={particle.id}
+            className="absolute w-3 h-3 animate-confetti"
+            style={{
+              left: `${particle.x}%`,
+              top: '-20px',
+              backgroundColor: particle.color,
+              animationDelay: `${particle.delay}s`,
+              animationDuration: `${particle.duration}s`,
+              transform: `rotate(${particle.rotation}deg)`,
+            }}
+          />
+        ))}
+      </div>
+
+      {/* Celebration card - click to dismiss */}
+      <button
+        type="button"
+        onClick={dismiss}
+        className="neo-card p-6 bg-[var(--color-neo-done)] animate-bounce-in pointer-events-auto cursor-pointer hover:scale-105 transition-transform focus:outline-none focus:ring-2 focus:ring-neo-accent"
+      >
+        <div className="flex flex-col items-center gap-4">
+          {/* Icons */}
+          <div className="flex items-center gap-2">
+            <Sparkles size={24} className="text-neo-pending animate-pulse" />
+            <PartyPopper size={28} className="text-neo-accent" />
+            <Sparkles size={24} className="text-neo-pending animate-pulse" />
+          </div>
+
+          {/* Avatar celebrating */}
+          <AgentAvatar name={agentName} state="success" size="lg" />
+
+          {/* Message */}
+          <div className="text-center">
+            <h3 className="font-display text-lg font-bold text-neo-text-on-bright mb-1">
+              Feature Complete!
+            </h3>
+            <p className="text-sm text-neo-text-on-bright/80 max-w-[200px] truncate">
+              {featureName}
+            </p>
+            <p className="text-xs text-neo-text-on-bright/60 mt-2">
+              Great job, {agentName}!
+            </p>
+          </div>
+
+          {/* Dismiss hint */}
+          <p className="text-xs text-neo-text-on-bright/40 mt-1">
+            Click or press Esc to dismiss
+          </p>
+        </div>
+      </button>
+    </div>
+  )
+}