T3 default-upstream PR association reproduction

This synthetic repository reproduces a T3 Code PR-association edge case without using any production repository or data.

The repository contains a merged pull request whose head branch is main and whose base is a maintenance branch. Separately, create a local feature branch that tracks origin/main:

git clone [email protected]:gsimone/t3-default-upstream-pr-repro.git
cd t3-default-upstream-pr-repro
git worktree add -b feature/local-worktree /tmp/t3-pr-repro-worktree origin/main
git -C /tmp/t3-pr-repro-worktree status --short --branch

Git records origin/main as the new feature branch's upstream. Before the T3 fix, PR resolution treated that upstream as the feature branch's published PR head, found the merged reverse PR for main, and reported it for feature/local-worktree. Once the thread became idle, the merged PR state caused the unrelated thread to auto-settle.

Expected behavior: an upstream default branch used as the starting point for a differently named local feature branch is a base relationship, not a PR association.