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/ActivityFeed.tsx

@@ -0,0 +1,93 @@
+import { Activity } from 'lucide-react'
+import { AgentAvatar } from './AgentAvatar'
+import type { AgentMascot } from '../lib/types'
+
+interface ActivityItem {
+  agentName: string
+  thought: string
+  timestamp: string
+  featureId: number
+}
+
+interface ActivityFeedProps {
+  activities: ActivityItem[]
+  maxItems?: number
+  showHeader?: boolean
+}
+
+function formatTimestamp(timestamp: string): string {
+  const date = new Date(timestamp)
+  const now = new Date()
+  const diffMs = now.getTime() - date.getTime()
+  const diffSec = Math.floor(diffMs / 1000)
+
+  if (diffSec < 5) return 'just now'
+  if (diffSec < 60) return `${diffSec}s ago`
+  if (diffSec < 3600) return `${Math.floor(diffSec / 60)}m ago`
+  return date.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' })
+}
+
+export function ActivityFeed({ activities, maxItems = 5, showHeader = true }: ActivityFeedProps) {
+  const displayedActivities = activities.slice(0, maxItems)
+
+  if (displayedActivities.length === 0) {
+    return null
+  }
+
+  return (
+    <div>
+      {showHeader && (
+        <div className="flex items-center gap-2 mb-2">
+          <Activity size={14} className="text-neo-text-secondary" />
+          <span className="text-xs font-bold text-neo-text-secondary uppercase tracking-wide">
+            Recent Activity
+          </span>
+        </div>
+      )}
+
+      <div className="space-y-2">
+        {displayedActivities.map((activity) => (
+          <div
+            key={`${activity.featureId}-${activity.timestamp}-${activity.thought.slice(0, 20)}`}
+            className="flex items-start gap-2 py-1.5 px-2 rounded bg-[var(--color-neo-bg)] border border-neo-border/20"
+          >
+            <AgentAvatar
+              name={activity.agentName as AgentMascot}
+              state="working"
+              size="sm"
+            />
+            <div className="flex-1 min-w-0">
+              <div className="flex items-center gap-2">
+                <span className="text-xs font-bold" style={{
+                  color: getMascotColor(activity.agentName as AgentMascot)
+                }}>
+                  {activity.agentName}
+                </span>
+                <span className="text-[10px] text-neo-text-muted">
+                  #{activity.featureId}
+                </span>
+                <span className="text-[10px] text-neo-text-muted ml-auto">
+                  {formatTimestamp(activity.timestamp)}
+                </span>
+              </div>
+              <p className="text-xs text-neo-text-secondary truncate" title={activity.thought}>
+                {activity.thought}
+              </p>
+            </div>
+          </div>
+        ))}
+      </div>
+    </div>
+  )
+}
+
+function getMascotColor(name: AgentMascot): string {
+  const colors: Record<AgentMascot, string> = {
+    Spark: '#3B82F6',
+    Fizz: '#F97316',
+    Octo: '#8B5CF6',
+    Hoot: '#22C55E',
+    Buzz: '#EAB308',
+  }
+  return colors[name] || '#6B7280'
+}