chore: prune stale remote-tracking refs on fetch

[seeones]

Summary

Enable automatic pruning of stale remote-tracking refs so the local clone stops accumulating zombie origin/* refs after GitHub deletes branches on merge.

Adds to the repository .gitconfig:

• fetch.prune = true — every fetch/pull now removes remote-tracking refs whose branch has been deleted on the remote (e.g. GitHub's "automatically delete head branches").
• push.autoSetupRemote = true — the first push of a new branch sets its upstream automatically.

Context

When a PR is merged with auto-delete enabled, GitHub deletes the remote branch but the local remote-tracking ref (refs/remotes/origin/<branch>) lingers until an explicit git fetch --prune. With fetch.prune this is now automatic. Note this prunes only the remote-tracking ref; the local branch itself is still kept (Git never deletes local branches implicitly) and shows [gone] as a signal to clean it up.

Testing

• git fetch --prune origin removed the two stale refs (origin/changeset-release/main, origin/fix/marketplace-tag-on-main).
• git config --show-origin --get fetch.prune resolves to the repository .gitconfig (included via global include.path), confirming the wiring.

Notes

• The repository .gitconfig is consumed via a global include.path entry; the file itself is tracked so the setting travels with the repo.
• No runtime code changes.