Plugin Conflicts Guardian: default pcg_guard_activation to true

[arthur791004]

Fixes #

Proposed changes

• Flip the default of the pcg_guard_activation filter from false to true, so the Plugin Conflicts Guardian activation probe and the install/update parse-error gate are on by default.
• Split the gate default in probe-endpoint.php so activation-mode probes inherit the new true default while update-mode probes (gated by pcg_guard_updates) keep defaulting to false.
• Update the feature README to reflect the new defaults.

Related product discussion/links

Does this pull request change what data or activity we track or use?

No.

Testing instructions

• Sync jetpack-mu-wpcom to a test site (no extra filter wiring needed).
• In wp-admin → Plugins, activate a known-good plugin: activation succeeds normally (probe runs, returns clean).
• Activate a plugin known to fatal on load: activation is blocked with the Plugin Conflicts Guardian admin notice naming the plugin.
• Upload-install a zip containing a PHP parse error: install is refused with a parse-error notice.
• Add add_filter( 'pcg_guard_activation', '__return_false' ); in an mu-plugin and confirm activations are no longer probed (back to core behavior).
• Confirm pcg_guard_updates flow remains off unless explicitly enabled — installing/updating a plugin does not trigger the post-update health-check + rollback path.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (WordPress.com Site Helper), and enable the try/pcg-guard-activation-default-true branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack-mu-wpcom-plugin try/pcg-guard-activation-default-true

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

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• ✅ Add changelog entries to affected projects
  

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/jetpack-mu-wpcom/src/features/plugin-conflicts-guardian/probe-endpoint.php	0/82 (0.00%)	0.00%	2 ❤️‍🩹

Full summary · PHP report

[taipeicoder]

As a follow-up PR, can we add logstash for activations that we managed to block?