fix: make playwright.yml workflow_call-only like its CI siblings

[josephfusco]

Summary

• playwright.yml had its own pull_request/push triggers and a concurrency block on top of being called as a job from ci.yml.
• Concurrency expressions in a called reusable workflow are evaluated in the caller's context, so ${{ github.workflow }} resolved to "CI" — the same value as ci.yml's own top-level concurrency group (CI-refs/heads/main). The nested playwright job collided with the group its own parent run already occupied, so GitHub cancelled it before it could run, which flipped the whole CI run to failure on nearly every push to main even though every visible job (phpcs, phpstan, phpunit, plugin-check, typos) passed. That's what was keeping the CI badge red.
• It also meant Playwright ran twice per push: once via ci.yml's uses: call, once via its own standalone trigger.
• Dropping the standalone triggers and concurrency block makes it workflow_call-only, matching phpcs.yml/phpstan.yml/phpunit.yml/plugin-check.yml/typos.yml, all of which are call-only with no concurrency block of their own.
• Confirmed via gh api .../branches/main/protection/required_status_checks that "Playwright"/"Playwright Tests" is not a required status check, so this doesn't affect branch protection.

Side effect: Playwright will now run on every PR into main via the ci.yml umbrella rather than only when **.php/**.ts/tests/e2e/**/etc. change, since per-job path filtering isn't supported inside a reusable-workflow call.

Test plan

• actionlint .github/workflows/playwright.yml passes
• CI run on this PR shows CI completing successfully (single Playwright execution, no spurious failure)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.90%. Comparing base (1ecd991) to head (825fcd6).

Additional details and impacted files

@@            Coverage Diff            @@
##               main      #32   +/-   ##
=========================================
  Coverage     63.90%   63.90%           
  Complexity       35       35           
=========================================
  Files             7        7           
  Lines           266      266           
=========================================
  Hits            170      170           
  Misses           96       96           

Flag	Coverage Δ
multisite	63.90% <ø> (ø)
phpunit	64.63% <ø> (ø)

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