feat: make GEMINI_AUTH=oauth authenticate in Docker

OAuth cannot be done interactively in the headless container, so the login
must be seeded into the mounted ~/.gemini. Three problems are fixed:

- Mount fragility on the Windows Docker Desktop target: the creds mount
  defaulted to ${HOME}/.config/javis/gemini, but ${HOME} is often unset when
  compose runs outside a WSL shell, silently mounting the wrong dir. Default is
  now the project-local ./docker/gemini-oauth (cross-platform), GEMINI_OAUTH_DIR
  still overrides.
- No visibility: when oauth is selected but no login is seeded, the path
  silently degraded to DDG/Brave. Added gemini_oauth_ready() + a one-time debug
  hint and a startup entrypoint warning (skipped on the browser role, fail-open).
- Seeding guidance: oauth_creds.json is the essential credential (refresh token;
  GOOGLE_GENAI_USE_GCA=true forces OAuth), which is what the readiness check and
  warning verify; docs recommend copying the whole ~/.gemini for convenience.

Adds docker/gemini-oauth/ seed dir (.gitkeep) with the login files gitignored,
GEMINI_OAUTH_DIR in .env.example, and updates DEPLOY.md, stream_browser_modes.md
and llm_contexts.md. Covered by 3 new tests (10 passed total).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker/entrypoint.sh

@@ -67,5 +67,19 @@ fi
 # --- Ensure the Piper voice exists (best effort) ---
 bash /app/docker/download-piper.sh || echo "[entrypoint] piper download failed; TTS may be unavailable"
 
+# --- Gemini OAuth login check (GEMINI_AUTH=oauth real-time search) ---
+# The browser-only role never runs the reply engine / web search, so skip the
+# check there. Otherwise warn (don't fail) when oauth is selected but no login
+# has been seeded into the mounted ~/.gemini, since the path silently degrades
+# to the DDG/Brave cascade.
+if [ "${JARVIS_ROLE:-full}" != "browser" ] \
+   && [ "${GEMINI_AUTH:-oauth}" = "oauth" ] \
+   && [ ! -f /root/.gemini/oauth_creds.json ]; then
+  echo "[entrypoint] 🔑 GEMINI_AUTH=oauth but no Gemini login at /root/.gemini/oauth_creds.json"
+  echo "[entrypoint]    Real-time search will fall back to DDG/Brave until you seed the login."
+  echo "[entrypoint]    Seed it: copy a logged-in ~/.gemini into the host's gemini-oauth dir"
+  echo "[entrypoint]    (default ./docker/gemini-oauth, or set GEMINI_OAUTH_DIR). See docs/DEPLOY.md."
+fi
+
 echo "[entrypoint] display=$DISPLAY ollama=$OLLAMA_BASE_URL whisper=$WHISPER_MODEL/$WHISPER_DEVICE"
 exec supervisord -c /app/docker/supervisord.conf