Add count command

[aruizacevedo]

Summary

• Adds a count command: node notes.js count prints You have N notes.
• Handles singular correctly (You have 1 note.)
• Updates the help line in the default case to include count

Test plan

• node notes.js count with no notes → You have 0 notes.
• Add 3 notes, run node notes.js count → You have 3 notes.
• Delete down to 1 note, run node notes.js count → You have 1 note.
• node notes.js (no args) shows count in the commands list
• node --test tests/notes.test.js — all 3 tests pass

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