Observability contracts: usage accounting, plan blocks, price table (Phase 0)

The 2026-09 observability wave's type-level foundation, landed before the
streams so nothing renders against guessed shapes:

- contracts/chat.ts: CallUsage + TurnUsage (per-model-call token accounting,
  one entry per teacher/wake turn keyed by messageId) and ChatRecord.turns —
  a DELIBERATELY parallel append-only usage log, not per-message metadata:
  the client re-POSTs its message list every turn and its copies never
  carry server-stamped fields, so anything inside messages gets clobbered
  by the next persist (the wave-3 merge-rule lesson).
- contracts/chat.ts: PlanBlockData — the persisted data-plan-block part
  (full DAG on plan_init, one-level neighborhood on plan updates, snapshot
  semantics so history renders what was true then).
- contracts/jobs.ts: JobUsage + JobRecord.usage — a sub-agent run's
  accumulated accounting, written by the runner.
- core/ports.ts + store/chats.ts: ChatStore.appendTurnUsage — idempotent
  per messageId (abort replays / wake retries append once); listChats no
  longer leaks turns into the list payload.
- core/prices.ts: PRICES + CONTEXT_LIMITS (generated from OpenRouter's
  published pricing via scripts/sync-pricing.ts) + pure folds: callCost,
  turnCost, sessionCost, contextWindow, jobCost. Unknown model or absent
  fields render as absent, never zero.
- server/observability.ts: barrel-approved surface for the transport layer
  (fixes the import-law violation in the graph status route that slipped
  through the wave-3 merge verification).

Capture wiring (agent onStepFinish -> persist seam, plan-block emission)
lands next as master seam work; the streams start against these shapes.

src/store/chats.ts

@@ -11,7 +11,7 @@ import * as fs from "node:fs";
 import * as path from "node:path";
 import { PATHS } from "../../config.ts";
 import type { ChatStore } from "../core/ports.ts";
-import type { ChatRecord, ChatSummary, PlanTree } from "../core/contracts.ts";
+import type { ChatRecord, ChatSummary, PlanTree, TurnUsage } from "../core/contracts.ts";
 import type { UIMessage } from "ai";
 import { findPendingToolCall } from "../core/pending.ts";
 import { phaseOfTree } from "../core/phases.ts";
@@ -51,7 +51,9 @@ function listChats(): ChatSummary[] {
 			.filter((f) => f.endsWith(".json"))
 			.map((f) => getChat(f.slice(0, -5)))
 			.filter((r): r is ChatRecord => r !== null)
-			.map(({ messages, tree, ...summary }) => ({
+			.map(({ messages, tree, turns, ...summary }) => ({
+				// turns stay out of the list payload (list = summaries; usage
+				// aggregates ride the full record, the picker never needs them).
 				...summary,
 				messageCount: messages.length,
 				// Record-derived status for the session picker: phase from the
@@ -76,6 +78,18 @@ function saveMessages(id: string, messages: UIMessage[]): void {
 	fs.writeFileSync(chatFile(id), JSON.stringify(record, null, "\t"), "utf-8");
 }
 
+/** Append one usage entry to the record's turn log — idempotent per
+ * messageId: abort replays and wake-turn retries persist the same turn
+ * twice; the second append is a no-op. */
+function appendTurnUsage(id: string, entry: TurnUsage): void {
+	const record = getChat(id);
+	if (!record) throw new Error(`Chat not found: ${id}`);
+	record.turns = record.turns?.some((t) => t.messageId === entry.messageId)
+		? record.turns
+		: [...(record.turns ?? []), entry];
+	fs.writeFileSync(chatFile(id), JSON.stringify(record, null, "\t"), "utf-8");
+}
+
 /** Persist the session plan tree (plan tools call this in execute). */
 function saveTree(id: string, tree: PlanTree): void {
 	const record = getChat(id);
@@ -92,4 +106,5 @@ export const chatStore: ChatStore = {
 	listChats,
 	saveMessages,
 	saveTree,
+	appendTurnUsage,
 };