Add `cortex count` command for quick memory count

[KD-K2N2]

Description

Add a simple cortex count command that shows memory count without the full cortex status output.

Expected behavior

$ cortex count
47

$ cortex count --by-type
decision: 18
context: 12
preference: 8
thread: 5
error: 2
learning: 2

$ cortex count --by-project
karnyx: 23
cortex: 14
the-production-line: 10

$ cortex count --json
{"total":47}

Good first issue

Very simple command — just API call + formatted output. ~20 lines of code.