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/KanbanBoard.tsx

@@ -1,16 +1,22 @@
 import { KanbanColumn } from './KanbanColumn'
-import type { Feature, FeatureListResponse } from '../lib/types'
+import type { Feature, FeatureListResponse, ActiveAgent } from '../lib/types'
 
 interface KanbanBoardProps {
   features: FeatureListResponse | undefined
   onFeatureClick: (feature: Feature) => void
   onAddFeature?: () => void
   onExpandProject?: () => void
+  activeAgents?: ActiveAgent[]
 }
 
-export function KanbanBoard({ features, onFeatureClick, onAddFeature, onExpandProject }: KanbanBoardProps) {
+export function KanbanBoard({ features, onFeatureClick, onAddFeature, onExpandProject, activeAgents = [] }: KanbanBoardProps) {
   const hasFeatures = features && (features.pending.length + features.in_progress.length + features.done.length) > 0
 
+  // Combine all features for dependency status calculation
+  const allFeatures = features
+    ? [...features.pending, ...features.in_progress, ...features.done]
+    : []
+
   if (!features) {
     return (
       <div className="grid grid-cols-1 md:grid-cols-3 gap-6">
@@ -34,6 +40,8 @@ export function KanbanBoard({ features, onFeatureClick, onAddFeature, onExpandPr
         title="Pending"
         count={features.pending.length}
         features={features.pending}
+        allFeatures={allFeatures}
+        activeAgents={activeAgents}
         color="pending"
         onFeatureClick={onFeatureClick}
         onAddFeature={onAddFeature}
@@ -44,6 +52,8 @@ export function KanbanBoard({ features, onFeatureClick, onAddFeature, onExpandPr
         title="In Progress"
         count={features.in_progress.length}
         features={features.in_progress}
+        allFeatures={allFeatures}
+        activeAgents={activeAgents}
         color="progress"
         onFeatureClick={onFeatureClick}
       />
@@ -51,6 +61,8 @@ export function KanbanBoard({ features, onFeatureClick, onAddFeature, onExpandPr
         title="Done"
         count={features.done.length}
         features={features.done}
+        allFeatures={allFeatures}
+        activeAgents={activeAgents}
         color="done"
         onFeatureClick={onFeatureClick}
       />