Add magit-style two-step rebase across all clients

[jennings]

Goal

Replace the current single-step badjuju.rebase (cursor picks source,
typed prompt picks destination) with a magit-style two-step chord flow:
the first chord picks a source revision and its scope flag (-s /
-r / -b); the second chord picks a destination and its insert
position (-d / --insert-after / --insert-before).

The rebase runs immediately. There is no interactive sequence buffer
(unlike git's interactive rebase) — jj makes mid-sequence conflicts and
undo trivial, so the magit "open a buffer, edit, commit" step is
unnecessary.

Hotkey table

Chord (magit)	Chord (vim)	Action
r s	rr s	Select source — jj rebase --source <cursor>
r r	rr r	Select source — jj rebase --revisions <cursor>
r b	rr b	Select source — jj rebase --branch <cursor>
r o	rr o	Apply — --destination <cursor>
r A	rr A	Apply — --insert-after <cursor>
r B	rr B	Apply — --insert-before <cursor>
x	x	Universal cancel — clears any pending operation

Helix (and other code-action-only clients) get the same six rebase
actions plus cancel via textDocument/codeAction.

While we're here, the squash S / SS cancel-branch goes away —
x becomes the universal cancel for any pending op. S / SS
reverts to unconditional "squash file at cursor".

Sub-tasks

• Server: replace badjuju.rebase with badjuju.rebase.source / .commit + badjuju.cancel
• Neovim: two-step rebase chord flow with universal cancel
• VS Code: two-step rebase chord keybindings with universal cancel
• Emacs: two-step rebase chord flow with universal cancel
• Helix: expose rebase code actions for the pending-state cycle

The four client tickets depend on the server ticket landing first.

Out of scope (follow-ups)

• Multi-revision source selection (visual range across log lines)
• Conflict-resolution UX after a rebase produces conflicts
• Showing a dry-run preview before the rebase runs

Plan: https://github.com/jennings/badjuju (local file
/Users/stephen/.claude/plans/let-s-plan-for-implementing-smooth-hedgehog.md).