PHP SDK: wire release pipeline

Follow-up from the experimental PHP SDK review.

The PHP SDK currently has pre-merge coverage, but it is not wired into the release/post-merge publishing path. Track the release pipeline work in one place before enabling PHP SDK publication.

## Missing pieces

- Add `sdk-php` to `.github/config/publish.yml` / `.github/workflows/publish.yml` so the publish workflow can select and run PHP SDK publishing.
- Add a PECL `package.xml` if PECL distribution is part of the PHP SDK release path.
- Add `.github/actions/php/post-merge/action.yml` for PHP SDK post-merge publishing/tagging behavior.
- Include `sdk-php` in the SDK loop in `.github/workflows/post-merge.yml` around the existing SDK auto-publish detection block near line 100.

## Notes

A combined ticket is intentional: these pieces should be designed together so PHP release behavior matches the other SDKs and does not get forgotten after the initial experimental cut.

Related: #3302
Initial PHP SDK work: #3235

Read .github/config/publish.yml, .github/workflows/publish.yml, .github/workflows/post-merge.yml around the SDK auto-publish detection block, and compare existing SDK release actions. Add the PHP SDK entries, the PECL package.xml if required, and .github/actions/php/post-merge/action.yml so PHP publishing and tagging are covered consistently before publication is enabled.

Written by the indexing model from the issue text.