feat: store client credentials in config

[HamidA1998]

Closes #2.

What changed

• permctl configure now also stores token, cert_path, and cert_key in the config file.
• core/client.New now reuses the stored credentials when creating the gRPC client:
  • Adds authorization: Bearer <token> metadata when token is configured.
  • Uses TLS when cert material is configured (CA bundle if only cert_path is set; mTLS client cert if both cert_path + cert_key are set).
• Config file permissions are now restricted to 0600 since it can contain secrets.

Validation

• go test ./...