bug: workspace diff/commit counts stay pinned to the wrong base after a PR targets a different branch

What I did Created a workspace off one branch, then created a PR from it targeting a different base than the one the workspace was originally branched from (e.g. a stacked PR onto another feature branch, or a hotfix PR targeting a release branch instead of main).

What I expected The workspace's diff/commit counts to compare against the PR's actual target branch after the PR is created.

What happened The workspace keeps its comparison base pinned to whatever branch it was originally created from. Once the PR's real base differs from that, diff/commit counts stay wrong (inflated or understated) instead of reflecting the PR's target.

Reproduction steps

1. Create a workspace from main.
2. Push and open a PR targeting a branch other than the workspace's original base (a stacked PR onto another feature branch, or a hotfix PR against a release branch).
3. Look at the workspace's diff/commit counts — they still reflect the original base, not the PR's target.

Platform Server-side comparison-base logic (packages/server), not platform-specific.

I have a fix in progress that pins the workspace's stored base ref to the PR's real target (qualified against origin) right after PR creation.