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