docs: use packaged fixture CLI examples

[rogerchappel]

Summary

• Update README CLI examples to use bundled synthetic fixture paths that work after package install.
• Add CLI tests for installed-user --name smoke and built-in fixture tuning paths.

Verification

• npm run release:check

Risk Level

Low. Changes are docs plus additive CLI tests; runtime behavior is unchanged.

Rollback Plan

Revert this PR to restore the previous README examples and remove the added tests.