feat: add smart_rebase algorithm for merged commit handling

[jd]

Adds smart_rebase() that detects merged PRs via the GitHub API and
handles them during rebase. If merged commits are found, it does a
single git rebase -i that both drops them and rebases onto trunk,
avoiding conflicts from PRs modified on GitHub before merge.

If no commits are merged, falls back to simple git pull --rebase.

Integrated into mergify stack push during its rebase step.

[jd]

This pull request is part of a stack:

1. feat: add smart_rebase algorithm for merged commit handling (#1098) 👈
2. feat: add mergify stack sync CLI command (#1099)
3. feat: add inline CI, review, and merge status to mergify stack list (#1100)
4. style: use compact Unicode symbols instead of emoji in CLI output (#1101)

[mergify]

Merge Protections

Your pull request matches the following merge protections and will not be merged until they are valid.

🔴 🤖 Continuous Integration

This rule is failing.

• all of:
  • check-success=ci-gate

🔴 👀 Review Requirements

This rule is failing.

• any of:
  • #approved-reviews-by>=2
  • author = dependabot[bot]
  • author = mergify-ci-bot
  • author = renovate[bot]

🔴 🔎 Reviews

This rule is failing.

• #review-requested = 0
• #review-threads-unresolved = 0
• #changes-requested-reviews-by = 0

🟢 Enforce conventional commit

Wonderful, this rule succeeded.

Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?:

🟢 📕 PR description

Wonderful, this rule succeeded.

• body ~= (?ms:.{48,})

[Copilot]

Pull request overview

Adds a new stack sync/rebase flow that queries GitHub to detect already-merged stack commits and, when present, performs a single interactive rebase that drops those commits while rebasing onto trunk. This is wired into mergify stack push and covered by a new test suite for sync status detection and the smart_rebase() decision logic.

Changes:

• Introduce mergify_cli/stack/sync.py with get_sync_status(), _write_drop_script(), and smart_rebase().
• Update mergify stack push rebase step to use smart_rebase() and log how many merged commits were dropped.
• Add comprehensive tests covering merged vs remaining commit classification and smart_rebase() behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
mergify_cli/stack/sync.py	Implements merged-commit detection via GitHub API and a drop-on-rebase mechanism using GIT_SEQUENCE_EDITOR.
mergify_cli/stack/push.py	Switches push-time rebase logic from git pull --rebase to smart_rebase().
mergify_cli/tests/stack/test_sync.py	Adds unit tests for sync status detection, drop script behavior, and smart_rebase() decision paths.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[jd]

this is not used in this PR.