fix(stt): route transcription inputs through the shared read guard

`transcribe_audio` reads a local file and hands it to the configured STT
provider — for the hosted providers (Groq, OpenAI, Mistral, xAI, ElevenLabs)
that ships the file's bytes to a third-party API. The same local-input read
guard was added to image-gen (587be5b5b) and xAI video-gen (104232979) to keep
the agent from feeding credential/secret stores to a provider, but STT was
missed.

Call `get_read_block_error(file_path)` at the top of `transcribe_audio`, before
validation/dispatch, so a `.env`, `auth.json`, `.anthropic_oauth.json`,
`mcp-tokens/`, etc. is refused up front instead of being transcribed (and, for
hosted providers, exfiltrated). This is defense-in-depth, not a security
boundary — the guard's own message says so — but it restores parity with the
image/video-gen tools.

Regression test: a `.env` file is refused with the shared read-guard message
before any provider dispatch (mutation-verified).

tests/tools/test_transcription_tools.py

@@ -2317,7 +2317,9 @@ class TestLocalBaseUrlNoApiKey:
         assert not _is_local_or_private_url("")
 
 
-# ============================================================================
+# =====================================================================
+
+
 # CAF (iMessage voice note) conversion tests
 # ============================================================================
 
@@ -2456,3 +2458,27 @@ class TestCafConversion:
 
         assert result["success"] is True
         mock_convert.assert_not_called()
+=======
+class TestTranscribeCredentialReadGuard:
+    """transcribe_audio must refuse credential/secret stores before dispatch."""
+
+    def test_transcribe_audio_blocks_credential_read(self, tmp_path):
+        """A ``.env`` (secret-bearing) file is refused up front, so its
+        plaintext is never shipped to an external STT provider — mirroring the
+        read guard added to image-gen (587be5b5b) and xAI video-gen
+        (104232979)."""
+        from tools.transcription_tools import transcribe_audio
+        from agent.file_safety import get_read_block_error
+
+        env_file = tmp_path / ".env"
+        env_file.write_text("OPENAI_API_KEY=sk-secret\n")
+
+        expected = get_read_block_error(str(env_file))
+        assert expected, "test setup: a .env file should be read-blocked"
+
+        result = transcribe_audio(str(env_file))
+
+        assert result["success"] is False
+        # The error is the shared read-guard message, not an audio-validation
+        # or provider error — proving the guard fired before dispatch.
+        assert result["error"] == expected

tools/transcription_tools.py

@@ -2144,6 +2144,15 @@ def _transcribe_prepared_audio(file_path: str, model: Optional[str] = None) -> D
           - "error" (str, optional): Error message if success is False
           - "provider" (str, optional): Which provider was used
     """
+    # Refuse to feed a credential / secret store (auth.json, .env, OAuth
+    # tokens, mcp-tokens/, ...) to an STT provider: an external provider would
+    # ship its plaintext contents to a third-party API. Mirrors the local-input
+    # read guard added to image-gen (587be5b5b) and xAI video-gen (104232979).
+    from agent.file_safety import get_read_block_error
+    blocked = get_read_block_error(file_path)
+    if blocked:
+        return {"success": False, "transcript": "", "error": blocked}
+
     # Apply common path validation before provider resolution so invalid files
     # cannot trigger provider setup or lazy installation. The remote-upload
     # size cap is enforced separately below, only for non-local providers.