docs: add CLI doctor error code documentation

[sandy191020]

Description

This PR adds documentation for the error codes generated by the Facetpack CLI doctor.

A new docs/error-codes.md file documents each existing error code (E0000–E0007), including:

What the error means

Common causes

How to fix it

The content is based on the current error-code mapping logic used in the CLI, so the documentation accurately reflects actual behavior.

Type of Change

• Bug fix
• New feature
• Performance improvement
• Documentation
• Refactoring
• Other (please describe)

Related Issues

Closes #10

Changes Made

Added docs/error-codes.md

Documented all existing CLI doctor error codes (E0000–E0007)

Included descriptions, causes, and fixes for each code

Testing

• Added/updated tests
• All tests pass (bun run test:all)
• Benchmarks unaffected (cd examples/benchmark && bun run bench)

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have added comments for complex logic
• I have updated documentation if needed
• My changes don't introduce new warnings

Screenshots (if applicable)

Not applicable only documentation is added

Additional Notes

This PR does not change any runtime or CLI logic only documentation has been added.