M02-L05 exercise — Challenge: save only what belongs

Start: M02-L01–04; disposable CP01 practice repository with an existing baseline.

In the disposable repository, create build-note.tmp containing “Harmless generated teaching output.” Make a useful README clarification and deliberately stage both files. Confirm a baseline commit already exists.

Inspect state, unstage only the temporary file, add an exact .gitignore entry, and review the final staged snapshot. Commit the intended README/ignore changes. Submit before/after state, final commit summary, and a paragraph distinguishing unstage, discard, and ignore. Preserve the harmless temporary file long enough to demonstrate that unstaging did not delete it.