[codex] store CLI credentials separately

[saiprasadchary]

Summary

• store endpoint, token, cert path, and cert key per profile in ~/.permify/credentials instead of the normal profile YAML
• update permctl configure to prompt for those values, validate the connection with them, and persist them separately from tenant/profile config
• load stored credentials whenever commands create a gRPC client, including bearer auth and optional mTLS support
• add focused tests for credential storage plus token/TLS client setup

Why

Issue #2 specifically asks for endpoint/token/cert path/cert key storage in ~/.permify/credentials. The current open PR (#20) persists secrets in the standard profile YAML; this patch keeps the existing profile workflow for tenant selection while moving secrets into the separate credentials store requested in the maintainer note.

Demo

• issue-2-credentials-demo.mp4

Validation

• /tmp/go/bin/go test ./...

/claim #2