feat: add credential storage for endpoint, token, and TLS certs

/claim #2

Summary

Implements persistent credential storage for the Permify CLI as described in #2.

What this PR does:

Credential storage at ~/.permify/credentials - Stores endpoint, API token, certificate path, and certificate key in a YAML file with restrictive permissions (0600 file, 0700 directory) for security
Updated configure command - Now prompts for API token, certificate path, and certificate key alongside the existing URL prompt. Pre-fills values from previously stored credentials for convenience
TLS and token authentication in gRPC client - The New() function now reads stored credentials and automatically configures:
- Client TLS certificates when cert_path and cert_key are provided
- TLS transport for https:// and grpcs:// endpoints
- Bearer token authentication using the existing secureTokenCredentials / nonSecureTokenCredentials types based on TLS state
Cross-platform support - Uses os.UserHomeDir() as fallback for home directory resolution (Windows USERPROFILE, macOS/Linux HOME)
Validation - Ensures certificate path and key are provided together; fails fast on invalid TLS key pairs
Unit tests - Tests for credential serialization/deserialization, missing file handling, and endpoint security detection

Files changed:

File	Change
`core/client/credentials.go`	New: credential storage with YAML serialization, OS-aware path resolution
`core/client/credentials_test.go`	New: 4 unit tests for credential operations
`core/client/grpc.go`	Updated: loads stored credentials, builds TLS/token dial options
`core/cli/configure.go`	Updated: prompts for token + certs, stores credentials before connecting
`cmd/permctl/permctl.go`	Updated: cross-platform home directory resolution

Credential file format (~/.permify/credentials):

endpoint: localhost:3478
token: your-api-token
cert_path: /path/to/cert.pem
cert_key: /path/to/key.pem

Closes #2