chore(release): resolve git remotes from URLs with env overrides

[hunger]

Previously the script hard-coded 'upstream' for the canonical pull and 'origin' for the fork push, which fails on clones that name those remotes differently. Resolve them by URL instead: pick whichever remote points to prefix-dev/pixi (preferring 'upstream'), and whichever remote points to $GITHUB_USER/pixi (preferring 'origin'). $GITHUB_USER falls back to 'gh api user' when unset. $UPSTREAM_REMOTE and $FORK_REMOTE override the detection entirely for non-standard setups. The script exits with an actionable error if a remote can't be found.

This ports over functionality from the build backend release script.

How Has This Been Tested?

Works-for-me(TM), YOLO.

If the script fails to detect things properly: The remotes can be forced to the old, hard-coded values by setting UPSTREAM_REMOTE=upstream and FORK_REMOTE=origin

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• [-] I have made corresponding changes to the documentation
• [-] I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.