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/agent/tools/plan.ts

@@ -13,7 +13,7 @@ import {
 	setCurrent,
 	updateNode,
 } from "../../core/plan-tree.ts";
-import { getChat, saveTree } from "../../store/chats.ts";
+import { chatStore } from "../../store/chats.ts";
 import type { TeacherToolContext } from "../tools.ts";
 
 /**
@@ -52,12 +52,12 @@ async function runPlanOp(
 	chatId: string,
 	op: (tree: PlanTree) => PlanTree,
 ): Promise<PlanToolResult> {
-	const record = getChat(chatId);
+	const record = chatStore.getChat(chatId);
 	if (!record) return { ok: false, error: `Chat not found: ${chatId}`, tree: emptyTree() };
 	const current = record.tree ?? emptyTree();
 	try {
 		const next = op(current);
-		saveTree(chatId, next);
+		chatStore.saveTree(chatId, next);
 		return { ok: true, tree: next };
 	} catch (err) {
 		if (err instanceof PlanTreeError) return { ok: false, error: err.message, tree: current };
@@ -81,7 +81,7 @@ export function planTools(ctx: TeacherToolContext): ToolSet {
 					.describe("Initial `requires` edges between the nodes. Edge direction: from = prerequisite, to = dependent."),
 			}),
 			execute: async ({ nodes, edges }): Promise<PlanToolResult> => {
-				const record = getChat(ctx.chatId);
+				const record = chatStore.getChat(ctx.chatId);
 				if (!record) return { ok: false, error: `Chat not found: ${ctx.chatId}`, tree: emptyTree() };
 				const existing = record.tree;
 				if (existing && existing.nodes.length > 0) {