feat: add Atlas PRD agent for product discovery

- src/prompts/atlas.ts — full Atlas system prompt (6-phase PM discovery flow)
- src/tools/prd.ts — finalize_prd tool that signals PRD completion
- src/agents/atlas.ts — Atlas agent config (Tier A, conversational)
- src/atlas.ts — atlasChat() multi-turn session handler
- server.ts — /atlas/chat, /atlas/sessions endpoints

Made-with: Cursor

dist/agents/index.d.ts

@@ -2,8 +2,10 @@ import '../prompts/orchestrator';
 import '../prompts/coder';
 import '../prompts/pm';
 import '../prompts/marketing';
+import '../prompts/atlas';
 import './orchestrator';
 import './coder';
 import './pm';
 import './marketing';
+import './atlas';
 export { AgentConfig, AGENTS, getAgent, allAgents, pick } from './registry';