cli: coloured diffs, repo commit view, build log steps #313

Diffs and logs at a terminal.

• Coloured unified diffs for repo commit, repo diff, mr diff, mr range-diff.
• repo commit as a view with signature state and checks (both are in the JSON but never printed). The subject prints twice today in plain output too: message is taken from after the payload's first blank line, which is the whole message.
• build log: step lines marked, the failing step red.

Spec: docs/specs/2026-10-01-cli-terminal-output-design.md