ci: reuse existing winget fork for publication

[muscariello]

Summary

Update the WinGet publication workflow to reuse an existing agntcy/winget-pkgs fork instead of calling GitHub endpoints the project app cannot use.

What Changed

• require a pre-created agntcy/winget-pkgs fork instead of attempting to create one on demand
• replace gh repo sync with a clone of the existing fork plus a fetch from upstream/master
• make upstream remote setup idempotent for fork clones
• stop hard-coding the fork default branch when cloning
• document the pre-created fork requirement in CONTRIBUTING.md

Validation

• reproduced the pre-fix failure on main: run 24722363681 failed on merge-upstream
• validated the updated workflow on the branch: run 24722727851 completed successfully
• successful branch run confirmed the existing-fork flow works end to end and exited cleanly because AGNTCY.shadictl 0.1.1 is already up to date on upstream/master

Refs #69

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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