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/AgentMissionControl.tsx

@@ -0,0 +1,121 @@
+import { Rocket, ChevronDown, ChevronUp, Activity } from 'lucide-react'
+import { useState } from 'react'
+import { AgentCard } from './AgentCard'
+import { ActivityFeed } from './ActivityFeed'
+import type { ActiveAgent } from '../lib/types'
+
+const ACTIVITY_COLLAPSED_KEY = 'autocoder-activity-collapsed'
+
+interface AgentMissionControlProps {
+  agents: ActiveAgent[]
+  recentActivity: Array<{
+    agentName: string
+    thought: string
+    timestamp: string
+    featureId: number
+  }>
+  isExpanded?: boolean
+}
+
+export function AgentMissionControl({
+  agents,
+  recentActivity,
+  isExpanded: defaultExpanded = true,
+}: AgentMissionControlProps) {
+  const [isExpanded, setIsExpanded] = useState(defaultExpanded)
+  const [activityCollapsed, setActivityCollapsed] = useState(() => {
+    try {
+      return localStorage.getItem(ACTIVITY_COLLAPSED_KEY) === 'true'
+    } catch {
+      return false
+    }
+  })
+
+  const toggleActivityCollapsed = () => {
+    const newValue = !activityCollapsed
+    setActivityCollapsed(newValue)
+    try {
+      localStorage.setItem(ACTIVITY_COLLAPSED_KEY, String(newValue))
+    } catch {
+      // localStorage not available
+    }
+  }
+
+  // Don't render if no agents
+  if (agents.length === 0) {
+    return null
+  }
+
+  return (
+    <div className="neo-card mb-6 overflow-hidden">
+      {/* Header */}
+      <button
+        onClick={() => setIsExpanded(!isExpanded)}
+        className="w-full flex items-center justify-between px-4 py-3 bg-[var(--color-neo-progress)] hover:brightness-105 transition-all"
+      >
+        <div className="flex items-center gap-2">
+          <Rocket size={20} className="text-neo-text-on-bright" />
+          <span className="font-display font-bold text-neo-text-on-bright uppercase tracking-wide">
+            Mission Control
+          </span>
+          <span className="neo-badge neo-badge-sm bg-white text-neo-text ml-2">
+            {agents.length} {agents.length === 1 ? 'agent' : 'agents'} active
+          </span>
+        </div>
+        {isExpanded ? (
+          <ChevronUp size={20} className="text-neo-text-on-bright" />
+        ) : (
+          <ChevronDown size={20} className="text-neo-text-on-bright" />
+        )}
+      </button>
+
+      {/* Content */}
+      <div
+        className={`
+          transition-all duration-300 ease-out overflow-hidden
+          ${isExpanded ? 'max-h-[500px] opacity-100' : 'max-h-0 opacity-0'}
+        `}
+      >
+        <div className="p-4">
+          {/* Agent Cards Row */}
+          <div className="flex gap-4 overflow-x-auto pb-4 scrollbar-thin">
+            {agents.map((agent) => (
+              <AgentCard key={`agent-${agent.agentIndex}`} agent={agent} />
+            ))}
+          </div>
+
+          {/* Collapsible Activity Feed */}
+          {recentActivity.length > 0 && (
+            <div className="mt-4 pt-4 border-t-2 border-neo-border/30">
+              <button
+                onClick={toggleActivityCollapsed}
+                className="flex items-center gap-2 mb-2 hover:opacity-80 transition-opacity"
+              >
+                <Activity size={14} className="text-neo-text-secondary" />
+                <span className="text-xs font-bold text-neo-text-secondary uppercase tracking-wide">
+                  Recent Activity
+                </span>
+                <span className="text-xs text-neo-muted">
+                  ({recentActivity.length})
+                </span>
+                {activityCollapsed ? (
+                  <ChevronDown size={14} className="text-neo-text-secondary" />
+                ) : (
+                  <ChevronUp size={14} className="text-neo-text-secondary" />
+                )}
+              </button>
+              <div
+                className={`
+                  transition-all duration-200 ease-out overflow-hidden
+                  ${activityCollapsed ? 'max-h-0 opacity-0' : 'max-h-[300px] opacity-100'}
+                `}
+              >
+                <ActivityFeed activities={recentActivity} maxItems={5} showHeader={false} />
+              </div>
+            </div>
+          )}
+        </div>
+      </div>
+    </div>
+  )
+}