modify fails after sync --prune: "unknown revision" for pruned merged branches #564

Version: gh stack v0.2.1

Summary

gh stack modify fails to open after gh stack sync --prune has pruned merged branches. It aborts while resolving branch SHAs for branches that --prune itself deleted, instead of skipping them the way view and rebase do.

Steps to reproduce

1. Have a stack with one or more merged PRs still listed (e.g. main <- merged-a <- merged-b <- active-c <- active-d).
2. Run gh stack sync --prune. The merged branches' local refs are deleted, but their entries remain in the stack metadata (.git/gh-stack) — this is the documented behavior ("Stack metadata is preserved so that rebase and display logic continue to work correctly").
3. Run gh stack modify.

Actual

✗ failed to apply modifications: building snapshot: resolving branch SHAs: failed to run git: fatal: ambiguous argument 'merged-a': unknown revision or path not in the working tree.

modify resolves every branch entry (including pruned-merged ones) by name via git, and dies on the first one whose ref no longer exists.

Expected

modify should tolerate pruned merged branches — skip them the same way view and rebase already do (e.g. ✓ Skipping merged-a (PR #NNN merged)), and open the TUI over the remaining branches.

Notes

• gh stack view and gh stack rebase work fine in this state; only modify chokes.
• The two commands are internally inconsistent: sync --prune produces a stack state that modify cannot read.
• Workaround: manually remove the merged branches' entries from .git/gh-stack.