feat(stream): true-mode browser-action core + Gemini scaffold + mode design

First increment of the STREAM_BROWSER real-time-info modes (true = browser,
false = Gemini):

- browse-search.mjs: drives the on-screen Chrome via CDP so the action shows on
  the broadcast. `search` returns the top Google results (title/url/snippet);
  `youtube` plays the first result. Verified live: real-time Seoul weather
  results, and IU 'Good Day' MV playback.
- .env.example: GEMINI_API_KEY / GEMINI_MODEL for the false-mode Gemini account.
- docs/stream_browser_modes.md: architecture + integration map (brain config,
  the two mode-gated tools, registry, design decisions) for the remaining wiring.

The Python brain wiring (config.py mode/gemini fields, browseAndSearch +
geminiSearch tools, registry, specs, llm_contexts) lands next - it needs a
running brain and a Gemini key to verify, rather than committing untested edits
into the 39k-line engine.

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

.env.example

@@ -58,8 +58,12 @@ CHROME_START_URL=about:blank
 # Screen-share + browser mode.
 #   true  = the bot may go Live (screen-share the VNC desktop) and drive the
 #           on-screen browser for real-time info (search / play / read screen).
-#   false = no screen share; voice only, real-time info via API/MCP tools.
+#   false = no screen share; voice only, real-time info via the Gemini API.
 STREAM_BROWSER=true
+# Gemini account (used for real-time info when STREAM_BROWSER=false). Get a key
+# at https://aistudio.google.com/app/apikey and paste it here.
+GEMINI_API_KEY=
+GEMINI_MODEL=gemini-2.0-flash
 
 # ---------------------------------------------------------------------------
 # VNC screen broadcast