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/core/ports.ts

@@ -21,6 +21,7 @@ import type {
 	NodeMapping,
 	PlanTree,
 	SessionCheckpoint,
+	TurnUsage,
 } from "./contracts.ts";
 
 // ─── ChatStore — the session record's home (data/chats) ─────────────────────
@@ -36,6 +37,10 @@ export interface ChatStore {
 	saveMessages(id: string, messages: UIMessage[]): void;
 	/** Persist the session plan tree (plan tools call this in execute). */
 	saveTree(id: string, tree: PlanTree): void;
+	/** Append one usage entry to the record's turn log (the money channel,
+	 * 2026-09): idempotent per messageId — a turn persisted twice (abort
+	 * replays, wake-turn retries) appends exactly once. */
+	appendTurnUsage(id: string, entry: TurnUsage): void;
 }
 
 // ─── GraphStore — the append-only evidence ledgers (data/graph) ──────────────