feat: add Smart Fridge shell and per-credential shell routing

Implement Samsung FridgeOS-themed shell (PLAN.md §3.3) with inventory
management, temperature controls, diagnostics, alerts, and other
appliance commands. Add per-credential shell routing so static
credentials can specify which shell to use via the `shell` config field,
passed through ssh.Permissions.Extensions.

Also extract shared ReadLine helper from bash to the shell package so
both shells can reuse terminal input handling.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/config/config_test.go

@@ -255,6 +255,33 @@ listen_addr = ":9090"
 	}
 }
 
+func TestLoadCredentialWithShell(t *testing.T) {
+	content := `
+[[auth.static_credentials]]
+username = "samsung"
+password = "fridge"
+shell = "fridge"
+
+[[auth.static_credentials]]
+username = "root"
+password = "toor"
+`
+	path := writeTemp(t, content)
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(cfg.Auth.StaticCredentials) != 2 {
+		t.Fatalf("static_credentials len = %d, want 2", len(cfg.Auth.StaticCredentials))
+	}
+	if cfg.Auth.StaticCredentials[0].Shell != "fridge" {
+		t.Errorf("cred[0].Shell = %q, want %q", cfg.Auth.StaticCredentials[0].Shell, "fridge")
+	}
+	if cfg.Auth.StaticCredentials[1].Shell != "" {
+		t.Errorf("cred[1].Shell = %q, want empty", cfg.Auth.StaticCredentials[1].Shell)
+	}
+}
+
 func TestLoadMissingFile(t *testing.T) {
 	_, err := Load("/nonexistent/path/config.toml")
 	if err == nil {