docs: call out the dormant extension service worker after a late install

[lianghua1]

What happened

I lost about an hour to this on Windows and figured it's worth writing down.

I installed the bsk CLI with the official PowerShell script, added the extension from the Chrome Web Store, and ran bsk doctor. Everything came back green except extension connected. The extension was installed, enabled, the connection toggle was on, and the port matched the daemon (52800). It just never connected: bsk status kept returning an empty browsers list.

The confusing part was the daemon log. It showed nothing at all from the extension. No rejected origin, no failed handshake, not even a dropped connection. My own manual WebSocket probes at 127.0.0.1:52800 (using the extension's origin) connected fine, so the daemon was clearly reachable and healthy.

What was actually going on: Chrome had been open the whole time, long before I installed the daemon and the extension. Its MV3 service worker never woke back up, and none of the things I tried nudged it:

• opening a new tab in Chrome
• chrome://restart
• navigating to chrome-extension://<id>/popup.html from the command line

The moment I fully quit Chrome and reopened it, the daemon logged browser connected within a second.

What this PR does

Adds a short troubleshooting note to AGENT_INSTALL.md Step 4 so the next person lands on the fix instead of the dead end. Docs only.

What it does not do

I deliberately left the reconnect logic alone. Whether the extension should arm a periodic reconnect so it recovers without a browser restart feels like a maintainer call, and I did not want to guess at the trade-off. If that sounds useful, I can open a separate issue with what I observed.