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>

docs/stream_browser_modes.md

@@ -59,7 +59,12 @@ entry) and falls back to the master flag so behaviour is unchanged.
     login (not API-key auth) and fails fast when no login exists rather than
     erroring on "no auth method". The CLI is resolved from `PATH` or
     `~/.local/bin/gemini`; install with `npm i -g @google/gemini-cli` and sign
-    in once via interactive `gemini` ("Sign in with Google").
+    in once via interactive `gemini` ("Sign in with Google"). In Docker the login
+    can't be done interactively in the headless container: seed it instead by
+    copying a logged-in `~/.gemini` into the project-local `docker/gemini-oauth`
+    bind mount (or set `GEMINI_OAUTH_DIR`); the container reads/refreshes the
+    token there. `gemini_oauth_ready()` gates an actionable log hint, and the
+    entrypoint warns on startup, when oauth is selected but no login is seeded.
   - `apikey`: the REST endpoint (`generativelanguage.googleapis.com`) via stdlib
     `urllib` with the `google_search` grounding tool - no SDK dependency.
 - Both Gemini paths and the browser path return the same