Add count command

[pppmppp]

What changed

• Added a count command to the notes CLI (node notes.js count)
• Prints You have N note(s). with correct singular/plural handling
• Updated the default usage line to include count

Why

Quick way to see how many notes exist without listing them all — useful when the list is long.

Test plan

• node notes.js count with no notes → You have 0 notes.
• Add a note, run count → You have 1 note. (singular)
• Add another, run count → You have 2 notes. (plural)
• npm test — all 3 existing tests pass

Reviewer notes

• The singular/plural logic (n === 1 ? "" : "s") is simple but worth a quick look
• No new tests were added for count — the command is a thin wrapper over store.all().length, but a test could be added if desired

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