implement edit mode

[schacon]

Similar to #11102, GitButler has an "edit mode" where you can essentially checkout a single commit (something like detached HEAD), make whatever changes you want and then pop back out of edit mode, rebasing child commits with the new parentage.

❯ but status
╭┄00 [Unassigned Changes]
┊
┊╭┄t6 [sc-branch-29] (#66)
┊●   38e9fac awesome change
┊◐   868c14b another change
┊◐   4698f1c my first change
├╯
┊
● 204e309 (common base) [origin/main] 2025-07-06 08:10:34 +0200 Merge pull request #10 from schacon/sc-description

❯ but edit 868c14b
Switching to edit mode on commit 868c14b - my first change

You are now in edit mode. Make any file changes and then save your new commit state by running `but edit finalize` or cancel with `but edit cancel`

Files changed in this commit:
-    po A app/views/bookmarks/index.html.erb
-    h0 M config/routes.rb
-    kx A test-file.md

❯ but status
You are in edit mode on commit 868c14b - my first change. 

Make any file changes and then save your new commit state by running `but edit finalize` or cancel with `but edit cancel`

Files changed in this commit:
-    po A app/views/bookmarks/index.html.erb
-    h0 M config/routes.rb
-    kx A test-file.md

❯ but edit finalize
Saved new state of commit 868c14b -> 204e309
Rebasing remaining commits...
  - 42b004 -> 6d82fcf worktree change v1
  - 047e7d -> 79497a awesome stuff