feat(media-use): use CLI free HeyGen usage (#2027)

* feat(media-use): use CLI free HeyGen usage

* fix(media-use): address #2027 R1 nits — gate cli-source header to OAuth, export origin constant

- X-HeyGen-Source is now sent only on OAuth (Bearer) requests, not API-key ones —
  the backend ignores it for API-key traffic (normal billing), so it was dead
  metadata there. buildAuthHeaders + heygenAuthHeaders + tests updated.
- Export HEYGEN_CLI_ORIGIN_HEADER ("X-HeyGen-Client-Origin") for future cli:<origin>
  consumers.
- Document the deliberate paid/X4 confirm-before-call decision on heygen.tts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H5k87mPZ4d6yiFwcWSb8Vv

* refactor(cli): drop unused origin-header export, dedup auth-client tests

Fallow flagged 5 findings on this PR:
- major: HEYGEN_CLI_ORIGIN_HEADER was exported but never emitted or
  imported — speculative dead code ("future consumers"). Remove it; a
  real consumer can add the constant when one exists.
- 4x minor duplication in client.test.ts: fold the repeated
  `.rejects.toSatisfy(auth-code)` assertion into expectAuthCode(), and the
  repeated try/catch scrubbed-message assertion into expectRejectionMessage().

No behavior change; auth/client tests still 17/17.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H5k87mPZ4d6yiFwcWSb8Vv

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

skills/media-use/audio/scripts/lib/heygen.test.mjs

@@ -0,0 +1,60 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { heygenAuthHeaders } from "./heygen.mjs";
+
+function withCleanHeygenEnv(fn) {
+  const previousApiKey = process.env.HEYGEN_API_KEY;
+  const previousHyperframesApiKey = process.env.HYPERFRAMES_API_KEY;
+  const previousConfigDir = process.env.HEYGEN_CONFIG_DIR;
+  try {
+    delete process.env.HEYGEN_API_KEY;
+    delete process.env.HYPERFRAMES_API_KEY;
+    delete process.env.HEYGEN_CONFIG_DIR;
+    return fn();
+  } finally {
+    if (previousApiKey === undefined) delete process.env.HEYGEN_API_KEY;
+    else process.env.HEYGEN_API_KEY = previousApiKey;
+    if (previousHyperframesApiKey === undefined) delete process.env.HYPERFRAMES_API_KEY;
+    else process.env.HYPERFRAMES_API_KEY = previousHyperframesApiKey;
+    if (previousConfigDir === undefined) delete process.env.HEYGEN_CONFIG_DIR;
+    else process.env.HEYGEN_CONFIG_DIR = previousConfigDir;
+  }
+}
+
+test("heygenAuthHeaders does not tag API-key requests as CLI traffic", () => {
+  withCleanHeygenEnv(() => {
+    process.env.HEYGEN_API_KEY = "hg_test";
+    // API-key requests use normal billing; the backend ignores the cli-source
+    // header for them, so it's not sent.
+    assert.deepEqual(heygenAuthHeaders(), {
+      "X-Api-Key": "hg_test",
+    });
+  });
+});
+
+test("heygenAuthHeaders tags OAuth requests as CLI traffic", () => {
+  withCleanHeygenEnv(() => {
+    const dir = mkdtempSync(join(tmpdir(), "heygen-cred-"));
+    try {
+      process.env.HEYGEN_CONFIG_DIR = dir;
+      writeFileSync(
+        join(dir, "credentials"),
+        JSON.stringify({
+          oauth: {
+            access_token: "at_test",
+            expires_at: "2099-01-01T00:00:00Z",
+          },
+        }),
+      );
+      assert.deepEqual(heygenAuthHeaders(), {
+        Authorization: "Bearer at_test",
+        "X-HeyGen-Source": "cli",
+      });
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+});