docs: add guidance for apps with persistent connections (SSE/WebSocket)

[mecampbellsoup]

Summary

• Adds a "Apps with Persistent Connections (SSE, WebSockets)" subsection under "Timeouts and Slow Pages" in skills/agent-browser/SKILL.md
• Explains why wait --load networkidle always times out on apps with SSE, WebSocket, or long-polling connections
• Provides alternative wait strategies: domcontentloaded, element waits, text waits, JS condition waits

Context

We hit this while using agent-browser to QA a Django app that maintains an SSE connection on every authenticated page (for real-time chat updates). wait --load networkidle timed out every time because Playwright's networkidle requires zero network activity for 500ms, which never happens with a persistent connection.

The current SKILL.md recommends networkidle in multiple code examples without noting this limitation. This affects any app using SSE, WebSockets, long-polling, or similar persistent connection patterns — which includes most real-time web apps (chat, dashboards, collaborative editors, notification systems).

Related to #479 (changing open default to domcontentloaded).

Test plan

• Verified the timeout behavior against a real SSE app
• Confirmed wait --load domcontentloaded works as a replacement
• Review: does this section belong here or in references/commands.md?

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[ctate]

Nice catch — I hit the same timeout on a test SSE server and confirmed all four alternatives work. Two small asks: update the networkidle code comment to something like # Wait for zero network activity for 500ms (times out if page uses SSE/WebSocket), and add a "see below" note on the paragraph right above your new section so readers get the heads-up before copy-pasting networkidle. Otherwise this is good to go!