feat: tabla apps en registry — modelo, parser, indexer y CLI

Agrega soporte completo para indexar aplicaciones del directorio apps/.
Cada app tiene un descriptor app.md con frontmatter YAML que el indexer
recoge automaticamente. Incluye migracion 004, modelo App, ParseAppMD,
ValidateApp, store CRUD con FTS5, y soporte en fn list/search/show.
Crea descriptores app.md para docker_tui, pipeline_launcher y metabase_registry.

registry/validate.go

@@ -161,6 +161,44 @@ func ValidateProposal(p *Proposal) *ValidationError {
 	return nil
 }
 
+// ValidateApp checks integrity rules for apps.
+func ValidateApp(a *App, knownFunctions, knownTypes map[string]bool) *ValidationError {
+	var errs []string
+
+	if a.Name == "" {
+		errs = append(errs, "name is required")
+	}
+	if a.Lang == "" {
+		errs = append(errs, "lang is required")
+	}
+	if a.Domain == "" {
+		errs = append(errs, "domain is required")
+	}
+	if a.Description == "" {
+		errs = append(errs, "description is required")
+	}
+
+	if a.DirPath != "" && strings.HasPrefix(a.DirPath, "/") {
+		errs = append(errs, "dir_path must be relative to registry root")
+	}
+
+	for _, ref := range a.UsesFunctions {
+		if !knownFunctions[ref] {
+			errs = append(errs, fmt.Sprintf("uses_functions references unknown function: %s", ref))
+		}
+	}
+	for _, ref := range a.UsesTypes {
+		if !knownTypes[ref] {
+			errs = append(errs, fmt.Sprintf("uses_types references unknown type: %s", ref))
+		}
+	}
+
+	if len(errs) > 0 {
+		return &ValidationError{ID: a.ID, Errors: errs}
+	}
+	return nil
+}
+
 // ValidateType checks integrity rules for types.
 func ValidateType(t *Type, knownTypes map[string]bool) *ValidationError {
 	var errs []string