feat: configurable conflict resolver for rebase conflicts

[abersnaze]

Summary

• Adds a new git-up.rebase.conflict-resolver config setting that specifies a command to invoke when rebase conflicts occur
• git-up builds a default prompt with full context (branch names, conflicted files, resolution steps) and substitutes it into the command's {prompt} placeholder
• If the resolver succeeds, git-up continues to the next branch; if it fails, the repo is left in the conflicted state for manual resolution

Usage

git config git-up.rebase.conflict-resolver "claude -p {prompt}"
# or
git config git-up.rebase.conflict-resolver "aider --message {prompt}"

Changes

• git_wrapper.py: Added suppress_pop flag to stasher() to prevent auto-unstash when leaving conflicted state; added UnresolvedConflictError exception
• gitup.py: New setting, _build_resolver_prompt(), _try_resolve_conflicts(), and integration into the rebase path
• tests/test_conflict_resolver.py: 4 tests covering resolver success, failure, no-resolver fallback, and prompt content

Test plan

• python -m pytest PyGitUp/tests/test_conflict_resolver.py -xvs — all 4 new tests pass
• python -m pytest PyGitUp/tests/ -x — full suite (40 tests) passes with no regressions
• Manual: configure git-up.rebase.conflict-resolver, create a conflict, verify git up invokes the resolver

🤖 Generated with Claude Code

[msiemens]

@abersnaze

Could you rebase this on the latest master branch? I realized that otherwise the test suite will run the OLD tests only (from master) and not executed new tests from your PR.

[abersnaze]

Rebased onto latest master!

Also caught a test isolation bug while I was at it — a few existing tests that expect RebaseError were silently passing when git-up.rebase.conflict-resolver is set in global config (the resolver would kick in and resolve the conflict instead of raising). Added gitup.settings['rebase.conflict-resolver'] = None to those tests so they're not sensitive to whatever's in the environment.