cli: rename ok gate helpers to check

[greygoody]

What to build

Rename the gate rendering and exit-code helpers so the canonical check command is reflected in code names. Keep all human output, JSON shape, and exit behavior unchanged.

Acceptance criteria

• render_ok, render_ok_json, and ok_exit_code are renamed to check-oriented names.
• check and ok still share the same command path.
• Human output, JSON output, and exit-code behavior remain unchanged.
• cargo fmt --all -- --check passes.
• cargo test --workspace passes.

Blocked by

None - can start immediately.