feat(google): support service-account auth for TTS + Imagen, fix false availability

Google's TTS and Imagen tools advertised service-account auth
(GOOGLE_APPLICATION_CREDENTIALS) but only ever authenticated with an API
key string, so users with a service-account JSON could not use either tool.
google_tts.get_status() also over-reported availability when the JSON was
set, then failed at execute() — a silent-availability bug.

Separately, both hand-rolled _load_dotenv parsers kept inline comments as
values, so after `cp .env.example .env` every keyed tool falsely reported
"available" with no real credentials.

Changes:
- Add tools/google_credentials.py: lazy google-auth Bearer-token helper.
- google_tts: authenticate via Cloud TTS Bearer token when only a service
  account is configured; make get_status() honest.
- google_imagen: route service-account auth to Vertex AI
  ({location}-aiplatform.googleapis.com) with project/location resolution,
  alongside the existing AI Studio API-key path.
- Fix both _load_dotenv parsers to strip inline comments (quote-aware).
- Add google-auth to requirements; document the new env vars in .env.example.
- .gitignore: never commit GCP service-account key files.

Verified locally with a real service account: TTS produced a valid MP3 and
Imagen produced a valid 1408x768 PNG via Vertex AI. Existing test suite
passes (2 unrelated pre-existing failures only).

Closes #131

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

tools/base_tool.py

@@ -30,6 +30,7 @@ def _load_dotenv() -> None:
     env_path = Path(__file__).resolve().parent.parent / ".env"
     if not env_path.is_file():
         return
+    import re
     with open(env_path, encoding="utf-8", errors="ignore") as f:
         for line in f:
             line = line.strip()
@@ -37,13 +38,19 @@ def _load_dotenv() -> None:
                 continue
             key, _, value = line.partition("=")
             key = key.strip()
-            value = value.strip().strip("'\"")
-            # Strip inline comments: VAR=value  # comment
-            # But only if the # is preceded by whitespace (avoid stripping from values like colors)
-            if "  #" in value:
-                value = value[:value.index("  #")].rstrip()
-            elif "\t#" in value:
-                value = value[:value.index("\t#")].rstrip()
+            value = value.strip()
+            # Quoted value: take the content inside the quotes verbatim.
+            if value[:1] in ("'", '"'):
+                quote = value[0]
+                end = value.find(quote, 1)
+                value = value[1:end] if end != -1 else value[1:]
+            else:
+                # Strip an inline comment ('#' at line start or after
+                # whitespace) so "VAR=   # note" yields "" not "# note".
+                match = re.search(r"(^|\s)#", value)
+                if match:
+                    value = value[: match.start()]
+                value = value.strip()
             if key and key not in os.environ:
                 os.environ[key] = value