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>

client.py

@@ -52,13 +52,25 @@ def get_playwright_headless() -> bool:
 
 # Feature MCP tools for feature/test management
 FEATURE_MCP_TOOLS = [
+    # Core feature operations
     "mcp__features__feature_get_stats",
     "mcp__features__feature_get_next",
+    "mcp__features__feature_claim_next",  # Atomic get+claim for parallel execution
     "mcp__features__feature_get_for_regression",
     "mcp__features__feature_mark_in_progress",
     "mcp__features__feature_mark_passing",
     "mcp__features__feature_skip",
     "mcp__features__feature_create_bulk",
+    "mcp__features__feature_create",
+    "mcp__features__feature_clear_in_progress",
+    # Dependency management
+    "mcp__features__feature_add_dependency",
+    "mcp__features__feature_remove_dependency",
+    "mcp__features__feature_set_dependencies",
+    # Parallel execution support
+    "mcp__features__feature_get_ready",
+    "mcp__features__feature_get_blocked",
+    "mcp__features__feature_get_graph",
 ]
 
 # Playwright MCP tools for browser automation
@@ -107,7 +119,12 @@ BUILTIN_TOOLS = [
 ]
 
 
-def create_client(project_dir: Path, model: str, yolo_mode: bool = False):
+def create_client(
+    project_dir: Path,
+    model: str,
+    yolo_mode: bool = False,
+    agent_id: str | None = None,
+):
     """
     Create a Claude Agent SDK client with multi-layered security.
 
@@ -115,6 +132,8 @@ def create_client(project_dir: Path, model: str, yolo_mode: bool = False):
         project_dir: Directory for the project
         model: Claude model to use
         yolo_mode: If True, skip Playwright MCP server for rapid prototyping
+        agent_id: Optional unique identifier for browser isolation in parallel mode.
+                  When provided, each agent gets its own browser profile.
 
     Returns:
         Configured ClaudeSDKClient (from claude_agent_sdk)
@@ -211,6 +230,16 @@ def create_client(project_dir: Path, model: str, yolo_mode: bool = False):
         playwright_args = ["@playwright/mcp@latest", "--viewport-size", "1280x720"]
         if get_playwright_headless():
             playwright_args.append("--headless")
+
+        # Browser isolation for parallel execution
+        # Each agent gets its own isolated browser context to prevent tab conflicts
+        if agent_id:
+            # Use --isolated for ephemeral browser context
+            # This creates a fresh, isolated context without persistent state
+            # Note: --isolated and --user-data-dir are mutually exclusive
+            playwright_args.append("--isolated")
+            print(f"   - Browser isolation enabled for agent: {agent_id}")
+
         mcp_servers["playwright"] = {
             "command": "npx",
             "args": playwright_args,