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/core/graph-context.ts

@@ -22,7 +22,7 @@
  */
 
 import type { GraphEdge, GraphNode } from "./contracts.ts";
-import { deriveNodeStates, getGraph, type DerivedGraph } from "./graph.ts";
+import { deriveNodeStates, type DerivedGraph } from "./graph.ts";
 import type { NodeStateClass } from "./contracts.ts";
 
 /** The section header the teacher is taught to recognize (50-graph-context.md). */
@@ -59,7 +59,7 @@ function isHotspot(node: GraphNode, state: NodeStateClass): boolean {
  * Render the context block, or null when there is nothing worth injecting
  * (no nodes; or a truncated graph whose selection came out empty).
  *
- * Pure: callers hand in exactly what getGraph() + deriveNodeStates() produce.
+ * Pure: callers hand in exactly what getGraph(graphStore) + deriveNodeStates() produce.
  */
 export function renderGraphContext(
 	nodes: GraphNode[],
@@ -162,11 +162,11 @@ function edgeChains(edges: GraphEdge[]): string[][] {
 // ─── the per-request entry point ─────────────────────────────────────────────
 
 /**
- * Build the graph context block for one request. Stateless — re-reads the
- * ledgers every call, so it self-updates as the graph grows mid-session.
- * Accepts an injected derived graph (tests); defaults to the live one.
+ * Build the graph context block for one request. Pure: the caller hands in
+ * the derived graph (production: getGraph(graphStore) per request in the
+ * chat-handler, so it self-updates as the graph grows mid-session).
  */
-export function buildGraphContext(graph: DerivedGraph = getGraph()): string | null {
+export function buildGraphContext(graph: DerivedGraph): string | null {
 	const states = deriveNodeStates(graph.nodes, graph.events);
 	return renderGraphContext(graph.nodes, graph.edges, states);
 }