tui 1/8: engine scaffold, `git queue tui` command & guards

[freshtonic]

Parent

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

What to build

The foundation for git queue tui: the subcommand exists, refuses to run in the wrong conditions, and can load the current queue line into a headless engine model — no interactive UI yet. git queue edit is untouched.

• The tui subcommand is wired into the CLI and dispatched.
• A headless engine (exposed from the library so tests can drive it in-process) holds the queue-line model: the ordered commit sequence (front/oldest → tip) and the branch boundaries over it, loaded by reusing edit's scope logic (current queue line; an untracked branch opens as one provisional section over trunk).
• The operations-as-data type is defined (reorder, squash, split, reword, delete, add/remove/move-boundary, rename-branch, undo, redo) even where variants are not yet handled.
• Guards: errors cleanly when stdout is not a TTY (pointing to git queue edit); requires a clean worktree; bails on an empty queue; refuses a forked line with a clear message (rewriting shared history is out of scope).

Acceptance criteria

• git queue tui is a recognised subcommand; git queue edit behaviour is unchanged.
• Running without a TTY exits non-zero with a message directing the user to edit.
• Running with a dirty worktree, an empty queue, or on a forked line each fails with a distinct, clear message.
• Engine integration test: loading a multi-branch queue line yields the correct commit sequence and boundary positions, with Stable-Commit-Ids read where present.
• Engine integration test: an untracked branch loads as a single provisional section over trunk.

Blocked by

• None — can start immediately.