feat: add mergify stack sync CLI command

[jd]

Exposes the smart_rebase algorithm as mergify stack sync, which
fetches trunk, detects merged PRs, drops them, and rebases in one
step. Supports --dry-run to preview what would happen.

Claude-Session-Id: 2d9f4a52-4a1d-4a50-a459-3b57847dec9f

[jd]

This pull request is part of a stack:

1. feat: add mergify stack sync CLI command (#1116) 👈
2. feat: add inline CI, review, and merge status to mergify stack list (#1117)

[mergify]

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[Copilot]

Pull request overview

Adds a new mergify stack sync workflow entrypoint to keep a local stack branch aligned with trunk by leveraging the existing smart_rebase logic (including dropping commits whose PRs have already been merged), plus documentation and integration tests to validate behavior.

Changes:

• Introduces stack_sync() in mergify_cli/stack/sync.py to fetch trunk, run the smart rebase flow, and print user-facing status (with --dry-run support).
• Wires a new mergify stack sync Click subcommand into the stack CLI.
• Adds integration-style tests for sync scenarios (up-to-date, merged commits dropped, all merged, dry-run) and updates the stack workflow skill docs.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
mergify_cli/stack/sync.py	Adds stack_sync() wrapper around fetch + smart_rebase() with user-facing output and --dry-run.
mergify_cli/stack/cli.py	Adds mergify stack sync command and forwards basic options to stack_sync().
mergify_cli/tests/stack/test_sync.py	Adds integration tests covering the new sync behavior and output.
mergify_cli/stack/skill.md	Documents the new mergify stack sync command and recommended usage.

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[mergify]

@jd this pull request is now in conflict 😩

Pull request has been modified.

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-04-10 18:34 UTC · Rule: default
• 🚫 Left the queue — 2026-04-10 18:34 UTC · at 7dc6609fcb8bd456007fe9e49dffb30d6cf0a1e1

This pull request spent 5 seconds in the queue, with no time running CI.

Reason

The pull request conflicts with pull requests ahead in queue: #1133

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There is nothing you can do for now. If the pull request ahead in the queue is merged, this pull request will become conflicting and you'll have to update it.
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[mergify]

@jd this pull request is now in conflict 😩

Pull request has been modified.