Let --checkup test for external executables

[rolandwalker]

Description

Let --checkup test for external executables:

• less
• fzf

and rework some formatting of the --checkup output.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

Findings

1. Docs correctness: --checkup is misspelled as --cheeckup in the changelog entry. That’s user-facing and misleading.
   File: changelog.md:6

2. Tests gap: New --checkup output (external executable checks and headings) isn’t covered by tests. If you have snapshot/CLI output tests elsewhere, add/update coverage to prevent regressions in the help/checkup output formatting.
   File: mycli/main.py:2354

If you want, I can draft a minimal test harness for --checkup output based on your existing CLI test patterns.