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/tools/prd.d.ts

@@ -0,0 +1,2 @@
+/** Shared state — atlas.ts reads this after each turn to check if PRD was finalized. */
+export declare const prdStore: Map<string, string>;