[STG-1862] feat: add browser-swarm skill

[shrey150]

Linear: https://linear.app/browserbase/issue/STG-1862/add-browser-swarm-skill

Summary

• Add a browser-swarm skill for same-browser multi-agent workflows using bb browse --auto-connect
• Document the target-bound CDP/Playwright/Stagehand fallback for true parallel tab ownership
• Register the skill in the README and browse plugin marketplace metadata

Verification

• node -e "JSON.parse(require('fs').readFileSync('.claude-plugin/marketplace.json','utf8')); console.log('marketplace json ok')"
• git diff --check
• git log --oneline origin/main..HEAD
• git diff --stat origin/main..HEAD

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Note

Low Risk
Primarily adds new documentation and a standalone helper script; risk is low and limited to potential confusion/misuse of the new experimental multi-agent workflow guidance.

Overview
Adds a new browser-swarm skill that documents an experimental same-Chrome, multi-tab “swarm” workflow using bb browse env local --auto-connect, including session ownership and safety/approval guardrails.

Introduces skills/browser-swarm/scripts/understudy-target.mjs, a small CLI helper that attaches Stagehand/Understudy to a shared CDP websocket and runs targetId-scoped actions (list pages, open a new page, click/fill/press/goto, extract text/html, take screenshots) to avoid “active tab” races.

Registers the new skill in README.md and updates .claude-plugin/marketplace.json to include the skill plus expanded browse plugin metadata/keywords around multi-agent tab coordination.

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