Call the shared PHPCS, PHPStan and AI checklist workflows [ignore_release]

[snake14]

Description

Pilot for the shared plugin CI workflows in matomo-org/plugin-ci-workflows, and the first public repository to call them. Replaces this plugin's own copies of the PHPCS, PHPStan and AI checklist workflows with calls to the shared definitions: 124 lines of duplicated CI removed for 15 lines of caller.

All three are pinned to the pull request branch of matomo-org/plugin-ci-workflows#2 so this pilot proves them on a runner before that merges. Once it does, the refs change to @main and workflows-ref drops out, leaving each caller as the shared workflow plus the plugin name.

No input other than plugin-name is needed here: this plugin is on the Matomo 5 line, which is what the shared defaults already target.

The shared PHPStan workflow analyses against the oldest supported Matomo as well as the newest, where the previous copy only checked the newest. Any new findings on that leg are the point of the change rather than a regression.

No version bump or changelog entry: this is CI-only.

Issue No

Related to PG-4897

Steps to Replicate the Issue

NA — CI change.

Checklist

• [✔] Tested locally or on demo2/demo3?
• [NA] New test case added/updated?
• [NA] Are all newly added texts included via translation?
• [NA] Are text sanitized properly? (Eg use of v-text v/s v-html for vue)
• [NA] Version bumped?
• [✔] I have understood, reviewed, and tested all AI outputs before use
• [✔] All AI instructions respect security, IP, and privacy rules
• [NA] Documentation updated?