feat: add jj spr cleanup command

[edmondop]

Summary

• New jj spr cleanup command that finds and deletes orphan SPR branches on the remote
• Queries GitHub GraphQL API for open PR branches, compares against remote refs to identify orphans
• Dry-run by default (lists orphans), --confirm flag to actually delete
• Includes GraphQL query with pagination, new get_open_pr_branch_names() GitHub method
• 6 unit tests for branch extraction and orphan filtering logic

Test plan

• cargo test — all tests pass
• cargo clippy --all-features --all-targets -- -D warnings — clean
• Run jj spr cleanup to list orphan branches
• Run jj spr cleanup --confirm to delete them
• Verify with git branch -r | grep spr/ that orphans are gone

[jennings]

Wonderful! All of these PRs you opened are like you've been watching the little paper cuts I keep getting but haven't had the time to fix.

This (and the others) seem good, I'll launch the workflows and look around, then merge them.

Thanks!