Split contracts.ts by domain: plan / graph / chat / jobs

Purely mechanical: src/core/contracts/{plan,graph,chat,jobs}.ts carry the
sections verbatim (graph = nodes + ledgers + extraction findings; jobs =
the async ack-and-steer protocol + tool results + demo bridge), and
contracts.ts becomes a re-exporting barrel so every existing import path
(@/src/core and ./contracts.ts) is unchanged.

Wave-2 lane ownership follows the split: the graph stream owns
contracts/graph.ts, the teacher stream owns contracts/jobs.ts — same-file
contract collisions between streams are structurally impossible now;
cross-domain schema changes go through master.

src/core/contracts/plan.ts

@@ -0,0 +1,61 @@
+/**
+ * Plan-tree contracts — the session-scoped dependency tree
+ *
+ * Part of the domain-split contracts (src/core/contracts/): each domain file
+ * is owned outright by its stream — cross-domain schema changes go through
+ * master. Pure types + protocol docs + tiny pure helpers; type-only imports
+ * of "ai" (safe everywhere: server, client, tests). See docs/vision.md.
+ */
+// ─── Plan tree (vision §2 "formal plan tree") ────────────────────────────────
+// A session-scoped dependency tree authored by the teacher via plan tools.
+// One substrate with the global graph: tree nodes fold into graph nodes.
+
+export interface PlanNode {
+	/** Opaque, agent-chosen slug, stable across edits: "chain-rule-intuition". */
+	id: string;
+	/** Short label for the tree panel. */
+	title: string;
+	/** 1-2 sentences: what this node covers. */
+	summary?: string;
+}
+
+export interface PlanEdge {
+	/** Prerequisite node id. */
+	from: string;
+	/** Node that depends on `from`. */
+	to: string;
+}
+
+export interface PlanTree {
+	nodes: PlanNode[];
+	edges: PlanEdge[];
+	currentNodeId: string | null;
+	doneNodeIds: string[];
+	/** Increments on every mutation — lets the UI animate/react to changes. */
+	version: number;
+}
+
+/** Derived, never stored: status of a node for the panel (vision §2). */
+export type PlanNodeStatus = "done" | "current" | "upcoming";
+
+export function planNodeStatus(tree: PlanTree, nodeId: string): PlanNodeStatus {
+	if (tree.currentNodeId === nodeId) return "current";
+	if (tree.doneNodeIds.includes(nodeId)) return "done";
+	return "upcoming";
+}
+
+/**
+ * Plan tools (W1 implements; server-side WITH execute, mutate the chat's
+ * stored tree, and return the full updated tree so the panel can render the
+ * latest state straight from the stream). Every tool returns PlanToolResult.
+ * Tool names: plan_init, plan_add_node, plan_update_node, plan_remove_node,
+ * plan_add_edge, plan_remove_edge, plan_set_current, plan_mark_done.
+ * The tree panel derives its display from the LATEST plan tool output in the
+ * transcript (live during streaming) — no separate state channel.
+ */
+export interface PlanToolResult {
+	ok: boolean;
+	error?: string;
+	tree: PlanTree;
+}
+