tui 5/8: reword (editable message pane, explicit-apply)

[freshtonic]

Parent

#1 — git queue tui — interactive queue/history editor (TUI)

What to build

Reword a commit's message from an editable pane. (Runs parallel to reorder — it needs the undo stack but not the conflict machinery, since a message-only rewrite replays descendants cleanly.)

• The top-right message pane becomes editable for the selected commit.
• Edits live in a local buffer; the reword applies only on an explicit action (Ctrl-S / confirm), which rewrites the commit (Stable-Commit-Id preserved) and rebases descendants (their ids preserved), pushing one undo entry.
• Navigating away with an unapplied buffer prompts apply / discard.

Acceptance criteria

• The message pane is editable for the selected commit.
• Typing does not touch git; only explicit apply rewrites the commit.
• After apply, the commit's Stable-Commit-Id is unchanged and descendants are rebased with their ids intact.
• Navigating away with unapplied edits prompts apply/discard.
• The reword is a single undoable operation.
• Engine integration test: reword preserves the id, rebases descendants, and is undoable.

Blocked by

• tui 3/8: boundary edits, undo/redo, quit-guard & exit #4 (ticket 3 — boundary edits, undo/redo & exit).