Automate OpenUPM releases with tags and changelog workflow

**What**

Implement CI/CD release automation for OpenUPM publishing using release tags, and add a workflow/PR automation for managing release tags and changelog updates.

**Why**

Manual release processes are error-prone and slow. Automated tagging and changelog generation improve release consistency and reduce maintenance overhead.

**How**

* Define release/tagging strategy compatible with OpenUPM package publishing
* Add GitHub workflow to handle release tag creation and release flow triggers
* Add changelog generation/update step as part of release preparation
* Optionally automate release-prep PR creation containing tag/changelog updates
* Ensure workflow permissions and branch protections are respected
* Document release process and rollback/retry guidance

**Acceptance Criteria**

* Release tags can be generated through automated workflow
* OpenUPM release process is automated or fully workflow-assisted
* Changelog is automatically generated/updated as part of release flow
* Release-prep PR automation is in place (if selected approach is implemented)
* Documentation describes end-to-end release process

**Estimation**

**Dependencies**

Part of #14

No files, tests, or entry points are named. Start by reviewing the repository's current release configuration and the OpenUPM publishing requirements, then define the tag, changelog, workflow, permissions, and rollback flow. Done means releases can be workflow-assisted or automated and the end-to-end process is documented.

Written by the indexing model from the issue text.