Storage ports: ChatStore/GraphStore/JobStore + adapters + core inversion (stage 3)

Per-entity semantic ports (src/core/ports.ts), one adapter family
(src/store/*, fs JSON/JSONL), exactly one composition point
(src/server/stores.ts). Dependency direction is now machine-checked:
adapters implement core interfaces; core never imports an adapter.

The inversion:
- core/graph.ts (emitChat/getGraph/graphSnapshot/foldStaleChats) is now
  pure — it takes the storage ports as explicit parameters (GraphDeps).
  No store imports; newGraphNodeId is local; node id format unchanged.
- core/graph-context.ts buildGraphContext() no longer defaults to the
  live ledgers: the chat-handler hands in getGraph(graphStore) per turn.
- store adapters export exactly one object each implementing their port
  (chatStore/graphStore/jobStore); bodies otherwise verbatim.
  NodeMapping + SessionCheckpoint moved store/graph -> core/contracts
  (they ARE contract shapes); CreateJobInit/JobPatch live in ports.
- titleFromPrompt moved store/chats -> core/contracts (pure helper,
  planNodeStatus precedent). Store modules now contain persistence only.
- server/stores.ts is the composition root; server/{chats,graph,jobs}.ts
  and chat-handler wire from it; agent (plan tools, sub-agent runner)
  call the port objects. The shell surface (@/src/server exports) is
  unchanged.

Lint law extended: the whole of src/core is now a purity zone (was a
file glob with a graph/graph-context exemption). Verified it fires on
core->store imports; tsc clean, 190/190 tests, eslint clean, prod build
clean (prerender 6/6). Tests pass deps explicitly — graph.test uses the
same fs-adapter pair stores.ts wires in production; graph-context.test
keeps its in-memory ledgers as a GraphStore implementation.

src/server/chats.ts

@@ -1,9 +1,11 @@
-import { createChat, getChat, listChats, titleFromPrompt } from "../store/chats.ts";
+import { stores } from "./stores.ts";
+import { titleFromPrompt } from "../core/contracts.ts";
 
 /**
  * Server-side chat entries for the transport layer (app/api/chats*,
- * app/chat/[id]). Today these delegate straight to the fs store; the
+ * app/chat/[id]). Today these delegate straight to the ChatStore port; the
  * services stage carves the real orchestration behind this same surface.
  */
 
-export { createChat, getChat, listChats, titleFromPrompt };
+export const { createChat, getChat, listChats } = stores.chatStore;
+export { titleFromPrompt };