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/styles/globals.css

@@ -870,6 +870,96 @@
   }
 }
 
+/* ============================================================================
+   Agent Mascot Animations
+   ============================================================================ */
+
+@keyframes bounce-gentle {
+  0%, 100% {
+    transform: translateY(0);
+  }
+  50% {
+    transform: translateY(-4px);
+  }
+}
+
+@keyframes thinking {
+  0%, 100% {
+    transform: translateY(0) scale(1);
+  }
+  25% {
+    transform: translateY(-2px) scale(1.02);
+  }
+  50% {
+    transform: translateY(0) scale(1);
+  }
+  75% {
+    transform: translateY(-2px) scale(0.98);
+  }
+}
+
+@keyframes working {
+  0%, 100% {
+    transform: translateX(0);
+  }
+  25% {
+    transform: translateX(-1px);
+  }
+  75% {
+    transform: translateX(1px);
+  }
+}
+
+@keyframes testing {
+  0%, 100% {
+    transform: rotate(0deg);
+  }
+  25% {
+    transform: rotate(-3deg);
+  }
+  75% {
+    transform: rotate(3deg);
+  }
+}
+
+@keyframes celebrate {
+  0%, 100% {
+    transform: scale(1) rotate(0deg);
+  }
+  25% {
+    transform: scale(1.1) rotate(-5deg);
+  }
+  50% {
+    transform: scale(1.15) rotate(0deg);
+  }
+  75% {
+    transform: scale(1.1) rotate(5deg);
+  }
+}
+
+@keyframes shake-gentle {
+  0%, 100% {
+    transform: translateX(0);
+  }
+  20%, 60% {
+    transform: translateX(-2px);
+  }
+  40%, 80% {
+    transform: translateX(2px);
+  }
+}
+
+@keyframes confetti {
+  0% {
+    transform: translateY(0) rotate(0deg);
+    opacity: 1;
+  }
+  100% {
+    transform: translateY(100vh) rotate(720deg);
+    opacity: 0;
+  }
+}
+
 /* ============================================================================
    Utilities Layer
    ============================================================================ */
@@ -970,6 +1060,35 @@
   .font-mono {
     font-family: var(--font-neo-mono);
   }
+
+  /* Agent mascot animation utilities */
+  .animate-bounce-gentle {
+    animation: bounce-gentle 2s ease-in-out infinite;
+  }
+
+  .animate-thinking {
+    animation: thinking 1.5s ease-in-out infinite;
+  }
+
+  .animate-working {
+    animation: working 0.3s ease-in-out infinite;
+  }
+
+  .animate-testing {
+    animation: testing 0.8s ease-in-out infinite;
+  }
+
+  .animate-celebrate {
+    animation: celebrate 0.6s ease-in-out;
+  }
+
+  .animate-shake-gentle {
+    animation: shake-gentle 0.5s ease-in-out infinite;
+  }
+
+  .animate-confetti {
+    animation: confetti 2s ease-out forwards;
+  }
 }
 
 /* ============================================================================