ci: add automated upstream sync workflow and mirror upstream releases on release branch pushes

[Copilot]

Adds GitHub Actions automation to keep this fork aligned with OpenStickCommunity/GP2040-CE and extends it to mirror upstream releases when release is pushed.

Changes Made

• Added upstream sync workflow (.github/workflows/sync-upstream.yml) to:
  • fetch upstream main
  • merge upstream changes when behind
  • push synced main updates to this repository
• Added push trigger for the release branch.
• Added release-mirroring job for release-branch pushes to:
  • sync upstream tags into this repository
  • create matching GitHub releases in this repository when missing
  • preserve release metadata (title/body, prerelease/draft flags)
• Implemented paginated upstream release retrieval so all upstream releases are considered, not just the first page.

Validation

• Workflow YAML validated successfully.
• Existing web build command completed successfully.
• Automated PR validation completed successfully (CodeQL/actions: no alerts).

[ap0ught]

@copilot make sure if we push to the release branch we will create the same releases as the upstream.

[Copilot]

@copilot make sure if we push to the release branch we will create the same releases as the upstream.

Implemented in 95599eb. The workflow now triggers on pushes to release, syncs upstream tags, and mirrors upstream GitHub releases into this repo (including pagination so all upstream releases are considered).