refactor(stack): add rebase helpers for squash and fixup

[jd]

Adds two helpers to mergify_cli/stack/reorder.py, used by the new
squash and fixup commands:

• run_action_rebase reorders picks to a desired sha order (same as
  run_rebase) and additionally rewrites pick -> squash|fixup for shas
  listed in actions. An optional exec_after_sha + exec_command pair
  inserts an "exec " line right after the named sha's row — used
  by squash to amend the combined commit's message while HEAD still
  points at it.
• display_action_plan prints the plan tagged with each commit's action.

run_rebase now delegates to run_action_rebase with an empty actions
dict to avoid duplicating the embedded rebase-todo rewrite script.

No user-facing behavior change — preparation for the fixup and squash
commands.

[jd]

This pull request is part of a Mergify stack:

#	Pull Request	Link
1	refactor(stack): add rebase helpers for squash and fixup	#1264	👈
2	feat(stack): add mergify stack fixup command	#1265
3	feat(stack): add mergify stack squash command	#1266

[mergify]

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[Copilot]

Pull request overview

Adds helper functions to support upcoming stack squash/fixup functionality by enabling action-aware interactive rebases and action-tagged plan display.

Changes:

• Introduces run_action_rebase() to reorder commits and rewrite rebase-todo actions (pick → squash/fixup) with optional exec insertion.
• Introduces display_action_plan() to print a commit plan annotated with each commit’s intended action.
• Minor docstring formatting adjustment in run_scripted_rebase().

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[jd]

Revision history

#	Type	Changes	Date
1	initial	eada573	2026-04-20 14:08 UTC
2	content	eada573 → 80b8633	2026-04-20 14:08 UTC

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-04-21 07:37 UTC · Rule: default
• ✅ Checks passed · on draft merge queue: embarking main (6411089), #1267 and #1264 together #1274
• ✅ Merged — 2026-04-21 07:43 UTC · at 80b863372d8c28ff965cd86b4cb7aca2df4ac1b2 · squash

This pull request spent 5 minutes 55 seconds in the queue, including 5 minutes 27 seconds running CI.

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