pr-branch-auto-delete - Wait for the PR header before reading branches - #10327

Created by Claude, will be reviewed by @fregante first.

Waits for the PR header's branch links to render before getBranches() reads them, the same way clean-conversation-headers already waits for them.

Fixes #10204

Test URLs

1. Open https://github.com/pulls
2. Click on any PRs you can merge (in repositories without native auto-delete)
3. Merge the PR

Checked

• getBranches() returns the right base and head on logged-out server-rendered markup of an open, a draft, a merged, a same-repo and a cross-repo PR, plus the Files tab. That includes the reported conversation-activity-filter - Move to top right  #10187. So the selectors themselves are fine, which is why I think this is a timing issue (header not rendered yet when init runs) and not a markup change.
• The extension builds and loads on a public PR logged out without new errors. The feature itself doesn't run logged out.

Not checked (reviewer tasks)

• I couldn't reproduce the error. It needs a logged-in session with merge rights and client-side navigation (PR list → PR), which can't be done in the sandbox.
• Please confirm on the Test URLs steps above that the branch gets deleted after merging and that the console no longer shows Expected element not found: .head-ref.