fix: ensure agents are removed from Mission Control UI on completion

Previously, agents that completed their work would remain visible in the
Mission Control UI until a manual page refresh. This occurred because
the AgentTracker._handle_agent_complete method silently dropped completion
messages when an agent wasn't tracked (e.g., due to missed start messages
from WebSocket connection issues).

Backend changes:
- Modified _handle_agent_complete in server/websocket.py to always emit
  completion messages, even for untracked agents
- Synthetic completions use agentIndex=-1 and agentName='Unknown' as
  sentinel values to indicate untracked agents

Frontend changes:
- Updated useWebSocket.ts to handle synthetic completions by removing
  agents by featureId when agentIndex is -1
- Added 30-minute stale agent cleanup as defense-in-depth for users who
  leave the UI open for extended periods
- Updated TypeScript types to allow 'Unknown' as valid agent name

Component updates:
- AgentAvatar.tsx: Added UNKNOWN_COLORS and UnknownSVG fallback for
  rendering unknown agents with a neutral gray question mark icon
- CelebrationOverlay.tsx, DependencyGraph.tsx: Updated interfaces to
  accept 'Unknown' agent names

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/components/CelebrationOverlay.tsx

@@ -4,7 +4,7 @@ import { AgentAvatar } from './AgentAvatar'
 import type { AgentMascot } from '../lib/types'
 
 interface CelebrationOverlayProps {
-  agentName: AgentMascot
+  agentName: AgentMascot | 'Unknown'
   featureName: string
   onComplete?: () => void
 }