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/KeyboardShortcutsHelp.tsx

@@ -0,0 +1,93 @@
+import { useEffect, useCallback } from 'react'
+import { X, Keyboard } from 'lucide-react'
+
+interface Shortcut {
+  key: string
+  description: string
+  context?: string
+}
+
+const shortcuts: Shortcut[] = [
+  { key: '?', description: 'Show keyboard shortcuts' },
+  { key: 'D', description: 'Toggle debug panel' },
+  { key: 'T', description: 'Toggle terminal tab' },
+  { key: 'N', description: 'Add new feature', context: 'with project' },
+  { key: 'E', description: 'Expand project with AI', context: 'with features' },
+  { key: 'A', description: 'Toggle AI assistant', context: 'with project' },
+  { key: 'G', description: 'Toggle Kanban/Graph view', context: 'with project' },
+  { key: ',', description: 'Open settings' },
+  { key: 'Esc', description: 'Close modal/panel' },
+]
+
+interface KeyboardShortcutsHelpProps {
+  onClose: () => void
+}
+
+export function KeyboardShortcutsHelp({ onClose }: KeyboardShortcutsHelpProps) {
+  const handleKeyDown = useCallback(
+    (e: KeyboardEvent) => {
+      if (e.key === 'Escape' || e.key === '?') {
+        e.preventDefault()
+        onClose()
+      }
+    },
+    [onClose]
+  )
+
+  useEffect(() => {
+    window.addEventListener('keydown', handleKeyDown)
+    return () => window.removeEventListener('keydown', handleKeyDown)
+  }, [handleKeyDown])
+
+  return (
+    <div
+      className="fixed inset-0 z-50 flex items-center justify-center bg-black/50"
+      onClick={onClose}
+    >
+      <div
+        className="neo-card p-6 max-w-md w-full mx-4"
+        onClick={(e) => e.stopPropagation()}
+      >
+        {/* Header */}
+        <div className="flex items-center justify-between mb-6">
+          <div className="flex items-center gap-2">
+            <Keyboard size={20} className="text-neo-accent" />
+            <h2 className="font-display text-lg font-bold">Keyboard Shortcuts</h2>
+          </div>
+          <button
+            onClick={onClose}
+            className="neo-btn p-1.5"
+            aria-label="Close"
+          >
+            <X size={16} />
+          </button>
+        </div>
+
+        {/* Shortcuts list */}
+        <ul className="space-y-2">
+          {shortcuts.map((shortcut) => (
+            <li
+              key={shortcut.key}
+              className="flex items-center justify-between py-2 border-b border-neo-border/30 last:border-0"
+            >
+              <div className="flex items-center gap-2">
+                <kbd className="px-2 py-1 text-sm font-mono bg-neo-bg rounded border border-neo-border shadow-neo-sm min-w-[2rem] text-center">
+                  {shortcut.key}
+                </kbd>
+                <span className="text-neo-text">{shortcut.description}</span>
+              </div>
+              {shortcut.context && (
+                <span className="text-xs text-neo-muted">{shortcut.context}</span>
+              )}
+            </li>
+          ))}
+        </ul>
+
+        {/* Footer */}
+        <p className="text-xs text-neo-muted text-center mt-6">
+          Press ? or Esc to close
+        </p>
+      </div>
+    </div>
+  )
+}