Add error code documentation page for cli doctor

[Inerska]

Description

Facetpack generates error codes (E0001, E0002, etc.) but there's
no documentation explaining what each code means.

What to do

1. Create docs/error-codes.md
2. List each error code with:
   • Description
   • Common causes
   • How to fix

Example format

## E0001 - Expected token not found
**Cause:** Missing punctuation like `}`, `)`, or `;`
**Fix:** Check for unclosed brackets or missing semicolons