feat: play the Nth YouTube result in browseAndPlay via an index arg

agents/llm.md promises "play the Nth video from the top", but browseAndPlay
only ever clicked the first result. Add an optional 1-based index argument
(default 1, backward-compatible) threaded to the Node helper, which now clicks
the Nth a#video-title and clamps to the number of results returned so asking
beyond the list plays the last available video instead of failing.

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

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

@@ -6,16 +6,24 @@ video, or clip.
 
 ### Behaviour
 
-- Public schema is a single required `query` string (what to play).
+- Public schema is a required `query` string (what to play) plus an optional
+  `index` integer (1-based position in the search results, counted from the top
+  of the list). `index` defaults to `1` (first result), so existing callers and
+  "play X" requests are unchanged; "play the 3rd video" / "play the second one"
+  map to `index=3` / `index=2`.
 - **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.
+  `bot/scripts/stream-test/browse-search.mjs <query> youtube <index>`, which
+  searches YouTube and plays the chosen result on display `:1`. The broadcast
+  captures that display, so the playback is what viewers see.
+  - The helper clicks the `index`-th `a#video-title` in the results list. The
+    index is clamped to the number of results actually returned, so asking for a
+    position beyond the list plays the last available result rather than failing.
+- Returns `success` with the played video's title (and the resolved `index`), or
+  a failure message if the helper/Chrome is unavailable. It does NOT make an LLM
+  call.
 
 ### Principles