feat(config): add config-as-code YAML import/export

Add declarative config-as-code support via YAML files. Monitors and
alerts can be exported, version controlled, and applied across instances.

- goupkeep export [-o file.yaml] dumps current state
- goupkeep apply -f file.yaml creates/updates to match desired state
- --dry-run shows planned changes without applying
- --prune deletes monitors/alerts not in the YAML
- Matching by name, alert references by name, nested group children
- CLI refactored to subcommands (apply, export, serve) with backward compat
- 24 tests covering apply, export, validation, round-trip idempotency

internal/config/validate.go

@@ -0,0 +1,88 @@
+package config
+
+import "fmt"
+
+var validMonitorTypes = map[string]bool{
+	"http":  true,
+	"push":  true,
+	"ping":  true,
+	"port":  true,
+	"dns":   true,
+	"group": true,
+}
+
+func Validate(f *File) error {
+	alertNames := make(map[string]bool, len(f.Alerts))
+	for _, a := range f.Alerts {
+		if a.Name == "" {
+			return fmt.Errorf("alert missing name")
+		}
+		if alertNames[a.Name] {
+			return fmt.Errorf("duplicate alert name %q", a.Name)
+		}
+		alertNames[a.Name] = true
+		if a.Type == "" {
+			return fmt.Errorf("alert %q: missing type", a.Name)
+		}
+	}
+
+	monitorNames := make(map[string]bool)
+	for _, m := range f.Monitors {
+		if err := validateMonitor(m, monitorNames, false); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func validateMonitor(m Monitor, names map[string]bool, nested bool) error {
+	if m.Name == "" {
+		return fmt.Errorf("monitor missing name")
+	}
+	if names[m.Name] {
+		return fmt.Errorf("duplicate monitor name %q", m.Name)
+	}
+	names[m.Name] = true
+
+	if !validMonitorTypes[m.Type] {
+		return fmt.Errorf("monitor %q: invalid type %q", m.Name, m.Type)
+	}
+
+	if m.Type == "group" && nested {
+		return fmt.Errorf("monitor %q: groups cannot be nested inside other groups", m.Name)
+	}
+
+	switch m.Type {
+	case "http":
+		if m.URL == "" {
+			return fmt.Errorf("monitor %q: url is required for type http", m.Name)
+		}
+	case "ping":
+		if m.Hostname == "" {
+			return fmt.Errorf("monitor %q: hostname is required for type ping", m.Name)
+		}
+	case "port":
+		if m.Hostname == "" {
+			return fmt.Errorf("monitor %q: hostname is required for type port", m.Name)
+		}
+		if m.Port == 0 {
+			return fmt.Errorf("monitor %q: port is required for type port", m.Name)
+		}
+	case "dns":
+		if m.Hostname == "" {
+			return fmt.Errorf("monitor %q: hostname is required for type dns", m.Name)
+		}
+	}
+
+	if m.Type == "group" {
+		for _, child := range m.Monitors {
+			if err := validateMonitor(child, names, true); err != nil {
+				return err
+			}
+		}
+	} else if len(m.Monitors) > 0 {
+		return fmt.Errorf("monitor %q: only groups can have nested monitors", m.Name)
+	}
+
+	return nil
+}