Add edit command to the notes CLI

[MishaLehotskyi]

Adds an edit <id> <text> command so an existing note's text can be changed in place.

• lib/store.js: new edit(id, text) — loads the store, finds the note by id, replaces its text, saves.
• notes.js: new edit case wired to store.edit, and the help line now lists it.

Merged main into the branch to resolve the add/add conflict in lib/store.js. npm test passes.

[MishaLehotskyi]

Claude caught the planted bug: store.edit() writes to the result of .find() without checking it exists, so editing a non-existent id crashes with TypeError: Cannot set properties of undefined — and CI stayed green because no test covers edit.