feat: CLI authentication — login, token storage, auto-refresh #86

Context

The current auth/oidc package provides only the OIDC/PKCE browser flow for token acquisition. It's incomplete — no token storage, no refresh, no HTTP client integration. No raystack project uses it.

Meanwhile, every raystack CLI (frontier, compass, guardian) connects to APIs without authentication, which only works in development/trusted environments. Production APIs require auth, and CLIs have no way to authenticate.

Proposed solution

A complete CLI auth package following the pattern of gh auth login, gcloud auth login, and terraform login:

// Login — opens browser, does OIDC, stores token
auth.Login(ctx, auth.Config{
    Issuer:       "https://accounts.google.com",
    ClientID:     "...",
    RedirectURL:  "http://localhost:5454",
})

// Token — reads stored token, refreshes if expired
token, err := auth.Token(ctx)

// Client — returns http.Client that auto-attaches the token
client := auth.Client(ctx)

Components

1. Token acquisition — browser-based OIDC with PKCE (existing code from dropped auth/oidc)
2. Token storage — secure local storage at ~/.config/raystack/<app>/token.json
3. Token refresh — auto-refresh expired tokens using refresh_token
4. HTTP client — http.RoundTripper that reads stored token and attaches to requests
5. CLI commands — LoginCmd() and LogoutCmd() for easy integration with cli.Execute()

Integration with salt/cli

cli.Execute(
    cli.Name("frontier"),
    cli.Commands(
        auth.LoginCmd(authConfig),
        auth.LogoutCmd(),
        userCmd, groupCmd,
    ),
)

// In commands:
func newListCmd() *cobra.Command {
    return &cobra.Command{
        RunE: func(cmd *cobra.Command, args []string) error {
            client := auth.Client(cmd)
            // make authenticated API calls
        },
    }
}

References

• GitHub CLI auth: https://github.com/cli/cli/tree/trunk/internal/authflow
• Previous auth/oidc package (dropped in salt evolution PR refactor: evolve salt into raystack service framework #85) had the PKCE flow