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/server/observability.ts

@@ -0,0 +1,12 @@
+/**
+ * Server-side observability surface — the barrel-approved exports the
+ * transport layer (app/api/*) reads for background-process status.
+ *
+ * The dock (2026-09 observability design) needs: extraction-pipeline runs
+ * (from the pipeline's module-level registry) and the store handles for
+ * title joins. app/ routes import ONLY from @/src/core and @/src/server —
+ * this file is where the deep imports surface, so routes stay clean.
+ */
+
+export { extractionStatuses, type ExtractionPhase } from "../agent/subagents/extraction.ts";
+export { chatStore } from "../store/chats.ts";