Move the 5.x-dev checks onto the shared Plugins CI umbrella [ignore_release]

[snake14]

Description

Brings 5.x-dev onto the shared Plugins CI umbrella that 6.x-dev already uses. ci.yml is the 6.x-dev caller: it replaces the standalone AI checklist and PHPCS workflows, and adds the PHPStan, license, compatibility, timezone safety and release date checks. It also adds the canonical pre-push hook, which verify-hook: true now enforces.

Also adds the weekly release caller. It won't run from this branch yet: GitHub only schedules or dispatches a workflow that exists on the default branch, which is 6.x-dev, and 6.x-dev doesn't have this caller yet. Once it does, its weekly branch sweep dispatches this one, and it can also be dispatched by hand against 5.x-dev.

Issue No

No GitHub issue.

Steps to Replicate the Issue

NA

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?