Skill: Verify CLI changes (example for Verify skill)

# Verifying a CLI change

The handle is direct invocation. The evidence is stdout/stderr/exit code.

## Pattern

1. Build (if the CLI needs building)
2. Run with arguments that exercise the changed code
3. Capture output and exit code
4. Compare to expected

CLIs are usually the simplest to verify — no lifecycle, no ports.

## Worked example

**Diff:** adds a `--json` flag to the `status` subcommand. New flag
parsing in `cmd/status.go`, new output branch.

**Claim (commit msg):** "machine-readable status output."

**Inference:** `tool status --json` now exists, emits valid JSON with
the same fields the human output shows. `tool status` without the flag
is unchanged.

**Plan:**