fix: use user-provided JWT generation logic

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.

github-app-token/SKILL.md

@@ -19,7 +19,7 @@ The following environment variables MUST be set before invoking this skill:
 
 If any variable is missing, stop and tell the user which ones are required.
 
-Requires `openssl`, `curl`, and `grep` (standard on macOS and Linux).
+Requires `openssl`, `curl`, `grep`, and `jq` (standard on modern environments).
 
 ## Steps