tui 7/8: squash (adjacent + cross-boundary, jj identity)

[freshtonic]

Parent

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

What to build

Squash a commit into its adjacent (older) neighbour, following jj's change-identity rules.

• Squash the selected commit into the one older (adjacent) to it; the engine rebases immediately (conflict escape hatch if needed).
• Identity (follow jj): the older commit keeps its Stable-Commit-Id; the newer is absorbed. Descriptions are combined — both non-empty prompts for a combined message to trim; if either is empty the other is used.
• Cross-boundary squash: squashing across a branch boundary auto-lands the result in the older branch; if that empties the newer branch, prompt to dissolve it (reusing the dissolve logic from delete).

Acceptance criteria

• Squash folds a commit into its older adjacent neighbour, keeping the older commit's Stable-Commit-Id.
• Combined-description prompt appears when both messages are non-empty; the non-empty one is used otherwise.
• A cross-boundary squash lands in the older branch and prompts to dissolve the newer branch when it empties.
• Squash is undoable.
• Engine integration tests cover adjacent squash (id + description), and cross-boundary squash with the dissolve path.

Blocked by

• tui 6/8: delete + empty-commit handling & branch dissolve #7 (ticket 6 — delete & empty-commit handling).