chore(usage): decompose services/usage.ts into per-provider usage/* leaves (999 → 253) (#8545)

* chore(usage): extract crof, nanogpt, qoder, opencode, deepseek, bailian, vertex, xiaomi-mimo, xai, github usage fetchers into usage/* leaves

Decompose services/usage.ts (god-file phase 1): move the remaining per-provider
usage fetcher/parser logic into co-located leaves under open-sse/services/usage/
so usage.ts becomes a thin dispatcher (imports + USAGE_FETCHER_PROVIDERS +
getUsageForProvider switch + __testing re-exports).

New leaves (each a pure data transform or independent fetcher, no orchestration):
  - usage/github.ts    getGitHubUsage, formatGitHubQuotaSnapshot, inferGitHubPlanName, shouldDisplayGitHubQuota
  - usage/crof.ts      getCrofUsage
  - usage/nanogpt.ts   getNanoGptUsage
  - usage/qoder.ts     getQoderUsage, parseQoderUserStatusUsage
  - usage/opencode.ts  getOpencodeUsage
  - usage/deepseek.ts  getDeepseekUsage
  - usage/bailian.ts   getBailianCodingPlanUsage
  - usage/vertex.ts    getVertexUsage
  - usage/xiaomi-mimo.ts getXiaomiMimoUsage
  - usage/xai.ts       getXaiUsage

usage.ts re-exports parseQoderUserStatusUsage (named) and threads every helper
the existing __testing contract exposes (usage-utils / usage-service-hardening /
qoder-usage-quota / xiaomi-mimo-selftrack / xai-usage / vertex-spend suites read
them from services/usage). External importers unchanged.

open-sse/services/usage.ts: 1065 -> 256 lines (cap 800).
npm run check:file-size: OK. typecheck:core: OK. eslint: clean. check:cycles: OK.

Added characterization tests (tests/unit/usage-<provider>-split.test.ts) that
import each leaf directly and pin its export surface + key edges, mirroring the
existing usage-quota-core-split / usage-scalars-split pattern.

* docs(changelog): add fragment for this PR

tests/unit/usage-opencode-split.test.ts

@@ -0,0 +1,18 @@
+// Characterization of the services/usage.ts opencode split (god-file decomposition): the
+// OpenCode / OpenCode Zen triple-window fetcher (getOpencodeUsage) moved into
+// services/usage/opencode.ts so usage.ts stays a thin dispatcher. Behavior-preserving move —
+// this locks the export surface and the no-key fail-open message; the window-shaping edges are
+// exercised via fetchOpencodeQuota stubs in opencode-quota-fetcher tests.
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+const O = await import("../../open-sse/services/usage/opencode.ts");
+
+test("module exposes getOpencodeUsage", () => {
+  assert.equal(typeof O.getOpencodeUsage, "function");
+});
+
+test("getOpencodeUsage returns a no-api-key message when the key is missing", async () => {
+  const r = (await O.getOpencodeUsage("conn", "")) as { message?: string };
+  assert.match(r.message ?? "", /OpenCode API key not available/);
+});