feat(arr): sequential merge and track pr linking

[jonathanlab]

TL;DR

Rewrote arr merge to merge PRs sequentially (one at a time), waiting for CI on each before proceeding. Also improved arr track to automatically detect and link existing PRs.

What changed?

Sequential merge flow

• PRs now merge one at a time instead of rebasing all to main upfront
• Waits for CI checks to complete before each merge (gh pr checks --watch)
• Only updates next PR's base to main after current PR successfully merges
• Falls back to auto-merge + polling if blocked by branch protection
• 30 minute timeout for CI wait

Auto-link PRs on track

• arr track now checks GitHub for an existing PR on the bookmark
• Shows "Linked: PR #X" when found, making it easier to recover state

How to test?

1. Create a stack of 2+ PRs with arr submit
2. Run arr merge and verify it:
   - Waits for CI on first PR
   - Merges first PR
   - Updates second PR's base to main
   - Waits for CI on second PR
   - Merges second PR
3. Test Ctrl+C during merge - verify stack stays intact (only completed merges affected)
4. Run arr untrack <branch> then arr track <branch> on a branch with an existing PR - verify it shows Linked: PR #X

Why make this change?

The previous merge flow rebased all PRs to main upfront, which caused issues:

• If interrupted, the stack was broken (all PRs pointing at main)
• Hard to recover from failures mid-merge
• Lost the visual stack on GitHub

[jonathanlab]

• feat(arr): sequential merge and track pr linking #439 sequential merge and track pr linking 👈
• main

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Merge from bottom to top, or use arr merge