feat: add AGENT_BROWSER_ALLOWED_ORIGIN env var for stream server security

[wsxiaoys]

Summary

Added AGENT_BROWSER_ALLOWED_ORIGIN environment variable to src/stream-server.ts to allow whitelisting WebSocket origins via regex. This enhances security by allowing users to restrict which origins can connect to the stream server.

Also updated:

• README.md and docs/src/app/streaming/page.tsx with documentation on how to use the new environment variable.
• cli/src/output.rs to include the new variable in the help output.
• skills/agent-browser/SKILL.md to list the new environment variable.

Test plan

• Verified that the server rejects connections from disallowed origins.
• Verified that the server allows connections from allowed origins (matching the regex).
• Verified that connections with no origin (e.g. CLI tools) and file:// origins are still allowed.
• Verified documentation updates.

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[vercel]

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[wsxiaoys]

fix #310

[wsxiaoys]

Kindly ping for review