Pitch

Codex parity. After a long session of edits, the user wants to see "what did we actually change?" in one place. `/diff` runs `git diff` against the snapshot taken at session start (the workspace mirror in `snapshot::repo`) and renders it inline with the colored-diff highlighting from #380.

`/diff ` accepts a turn number or a relative duration (`5m`, `1h`).
`/diff staged` only shows currently-staged changes.
`/diff word` switches to word-level highlighting for prose / config edits.

Acceptance

• `/diff` (no args) shows everything changed in this session.
• Output uses the same +/- coloring as v0.8.6 feat(tui): inline diff highlighting in apply_patch / edit_file results #380.
• File-by-file collapsible.
• Standard verification gates pass.