Add ui-test-wpcom-token option to auto sign-in WP.com sites

[crazytonyli]

Description

Similar to #25325, this PR implements auto-sign-in WP.com sites using launch arguments.

This PR adds documentation about the auto-sign-in capability to AGENTS.md, so that your coding agents can know how to sign in to test sites and perform end-to-end tests.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 26.8. This milestone is due in less than 4 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

Generated by 🚫 Danger

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	31488
	Version	PR #25379
	Bundle ID	com.jetpack.alpha
	Commit	b4ceb59
	Installation URL	0otug9k5v58l8

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	31488
	Version	PR #25379
	Bundle ID	org.wordpress.alpha
	Commit	b4ceb59
	Installation URL	20e5106ujuqj8

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[wpmobilebot]

🤖 Build Failure Analysis

This build has failures. Claude has analyzed them - check the build annotations for details.

[kean]

Nice!