fix: store permify credentials in ~/.permify/credentials

[tanayv77]

/claim #2

Summary

• store endpoint, token, cert path, and cert key in a dedicated per-profile ~/.permify/credentials file
• keep the existing tenant/profile config focused on non-secret state
• prompt for optional credential fields during permctl configure and load them when building the gRPC client
• add regression tests proving the main config excludes credentials while load/write still reconstructs the full client config

Testing

• go test ./...`n
  Closes Endpoint, Token, Cert Path, and Cert Key Storage for CLI Tool #2

[tanayv77]

Follow-up update: PR #38 now also masks the access token during permctl configure, while keeping the maintainer-requested dedicated ~/.permify/credentials storage model. Validation still passes with go test ./....