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/ViewToggle.tsx

@@ -0,0 +1,46 @@
+import { LayoutGrid, GitBranch } from 'lucide-react'
+
+export type ViewMode = 'kanban' | 'graph'
+
+interface ViewToggleProps {
+  viewMode: ViewMode
+  onViewModeChange: (mode: ViewMode) => void
+}
+
+/**
+ * Toggle button to switch between Kanban and Graph views
+ */
+export function ViewToggle({ viewMode, onViewModeChange }: ViewToggleProps) {
+  return (
+    <div className="inline-flex rounded-lg border-2 border-neo-border p-1 bg-white shadow-neo-sm">
+      <button
+        onClick={() => onViewModeChange('kanban')}
+        className={`
+          flex items-center gap-1.5 px-3 py-1.5 rounded-md font-medium text-sm transition-all
+          ${viewMode === 'kanban'
+            ? 'bg-neo-accent text-white shadow-neo-sm'
+            : 'text-neo-text hover:bg-neo-neutral-100'
+          }
+        `}
+        title="Kanban View"
+      >
+        <LayoutGrid size={16} />
+        <span>Kanban</span>
+      </button>
+      <button
+        onClick={() => onViewModeChange('graph')}
+        className={`
+          flex items-center gap-1.5 px-3 py-1.5 rounded-md font-medium text-sm transition-all
+          ${viewMode === 'graph'
+            ? 'bg-neo-accent text-white shadow-neo-sm'
+            : 'text-neo-text hover:bg-neo-neutral-100'
+          }
+        `}
+        title="Dependency Graph View"
+      >
+        <GitBranch size={16} />
+        <span>Graph</span>
+      </button>
+    </div>
+  )
+}