chore: fix changeset package name

[marandaneto]

💡 Motivation and Context

The release workflow failed because the pending changeset referenced the Composer package name (posthog/posthog-php) instead of the Changesets workspace package name (posthog-php). Changesets could not find that package in the workspace, so pnpm changeset version exited before creating the release.

💚 How did you test it?

• Ran pnpm install --frozen-lockfile
• Ran pnpm changeset status and confirmed posthog-php is detected for a minor bump

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

An agent inspected the failed release logs, identified the mismatch between the changeset package name and package.json, and updated the existing changeset to reference posthog-php. Validation used Changesets status output rather than creating a new changeset, because this PR only fixes release metadata for an already pending changeset.