feat: replace SpawnDetector with MCP server for spawn1337 protocol

Instead of parsing terminal output for <spawn1337> tags, spawn capabilities
are now exposed as native MCP tools that Claude Code can call directly.
The MCP server (stdio transport) proxies requests to the existing REST API.

- Add src/mcp-server.ts with 6 tools: spawn_agent, list_agents,
  get_agent_status, get_agent_result, send_agent_message, cancel_agent
- Remove src/spawn-detector.ts and all references in session.ts/server.ts
- Add CLAUDEMAN_API_URL env var propagation to sessions and screens
- Write .mcp.json to case directories during creation
- Remove spawn1337 tag documentation from case-template.md
- Add claudeman-mcp bin entry to package.json

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

src/spawn-types.ts

@@ -212,7 +212,7 @@ export interface AgentStatusReport {
   dependencyStatus: 'waiting' | 'ready' | 'n/a';
 }
 
-// ========== Tracker State (for SpawnDetector) ==========
+// ========== Tracker State (for SpawnOrchestrator) ==========
 
 export interface SpawnTrackerState {
   enabled: boolean;