feat: store endpoint/token/certs in config

[arisnachy]

Summary

• store endpoint, token, cert path, and cert key per profile in ~/.permify/credentials
• keep normal profile config free of connection secrets while retaining the selected tenant/profile workflow
• load stored credentials into config.CliConfig before command clients are created
• add focused config tests for separate credential writes, credential merging on load, and IsConfigured with credential-backed endpoints

Validation

• go test ./...
• go build -o C:\tmp\permctl-pr24-test.exe ./cmd/permctl
• git diff --check

/claim #2