feat(brain): wire STREAM_BROWSER real-time modes into the reply engine (browser + Gemini)

Completes the two info modes in the Python brain:

- config.py: read STREAM_BROWSER / GEMINI_API_KEY / GEMINI_MODEL from env into
  Settings (stream_browser, gemini_api_key, gemini_model). Verified load_settings
  reads both modes.
- realtime_search.py: two fail-open backends returning the same fenced
  UNTRUSTED-WEB-EXTRACT envelope: browser_search() shells the Node CDP helper to
  drive the on-screen Chrome (visible on the broadcast); gemini_search() calls
  the Gemini REST API with google_search grounding.
- web_search.run(): routes by mode before the DDG cascade (true->browser,
  false->Gemini), falling through to DDG/Brave/Wikipedia on any miss.
- browse_and_play tool: plays a YouTube video on the shared screen (true mode
  only); registered in the tool registry.
- specs + docs/llm_contexts.md updated (new Gemini LLM context); CLAUDE.md spec
  registry updated.

Verified live against the running Chrome: true-mode webSearch returned real
Google results for "오늘 서울 날씨", browseAndPlay played the IU 밤편지 MV, and
false-mode degrades gracefully on a bad/absent key. A valid GEMINI_API_KEY is
still needed to confirm the real Gemini grounding output.

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

src/jarvis/tools/builtin/browse_and_play.spec.md

@@ -0,0 +1,25 @@
+## browseAndPlay Tool Spec
+
+Plays a YouTube video on the shared screen so it appears on the Go-Live
+broadcast. Used when the user asks the assistant to play / watch a song, music
+video, or clip.
+
+### Behaviour
+
+- Public schema is a single required `query` string (what to play).
+- **Mode-gated**: only acts when `STREAM_BROWSER` is true (`cfg.stream_browser`).
+  In voice-only mode (false) there is no screen to show, so it returns a short
+  message and does nothing.
+- Drives the on-screen Chrome by subprocessing the Node CDP helper
+  `bot/scripts/stream-test/browse-search.mjs <query> youtube`, which searches
+  YouTube and plays the first result on display `:1`. The broadcast captures
+  that display, so the playback is what viewers see.
+- Returns `success` with the played video's title, or a failure message if the
+  helper/Chrome is unavailable. It does NOT make an LLM call.
+
+### Principles
+
+- The Node layer owns Chrome/CDP; the Python tool only shells out to it, so the
+  brain stays free of a browser dependency.
+- Fail-open and explicit: any error returns a plain failure message rather than
+  raising into the reply loop.