Replaced my bash implementation with the user's provided snippet.
Key differences that fix the bad credentials issue on macOS:
1. Uses openssl enc -base64 -A instead of openssl base64
2. Uses jq -r -c . to strictly format the JSON header/payload
3. Explicitly wraps the RSA signature binary in b64enc.
Replaced generate_jwt.py with generate_jwt.sh using only openssl and
coreutils. Updated SKILL.md to remove the python fallback section and
use grep/cut for JSON parsing instead of python3.