docs: add usage examples to --help output

[ded-furby]

Summary

Add an examples section to the CLI help output and cover it with a focused help-output test.

Why this change

Issue #542 notes that cve-lite --help lists flags but does not show common invocations. Adding examples makes the CLI easier to discover for new users.

What changed

• added an Examples: section to printHelp() with common scan and advisory sync commands
• kept the examples aligned with current CLI behavior and wording from the issue
• added a test that asserts the examples block is present in the rendered help output

Validation

• npm test
• npm run build

User-facing impact

Does this change:

• affect scanning behavior
• affect output formatting
• affect JSON output
• affect docs only

Notes

Closes #542