feat: $B watch — passive observation mode

Claude enters read-only mode and captures periodic snapshots (every 5s)
while the user browses. Mutation commands (click, fill, etc.) are
blocked during watch. $B watch stop exits and returns a summary with
the last snapshot.

Requires headed mode ($B connect). This is the inverse of the scout
pattern — the workspace agent watches through the browser instead of
the sidebar relaying to it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.agents/skills/gstack-browse/SKILL.md

@@ -458,6 +458,7 @@ Refs are invalidated on navigation — run `snapshot` again after `goto`.
 |---------|-------------|
 | `chain` | Run commands from JSON stdin. Format: [["cmd","arg1",...],...] |
 | `inbox [--clear]` | List messages from sidebar scout inbox |
+| `watch [stop]` | Passive observation — periodic snapshots while user browses |
 
 ### Tabs
 | Command | Description |