v0.8.5: Extension composability — slots, contributes, lib.require relaxation

Manifest declarations for slots (host surfaces declare injection points)
and contributes (extensions declare UI contributions). lib.require()
relaxed from library-only to any package with exports. Admin slots
aggregation endpoint. SDK renderAll() helper. 7 new tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/handlers/extensions.go

@@ -37,6 +37,44 @@ var validTiers = map[string]bool{
 	models.ExtTierSidecar:  true,
 }
 
+// ValidateComposabilityFields checks that manifest slots and contributes
+// fields follow the expected conventions. Slots values must have a
+// "description" string. Contributes keys must follow "pkg:slot" naming.
+// Returns nil if valid or missing; returns an error string otherwise.
+func ValidateComposabilityFields(manifest map[string]any) string {
+	// Validate slots: map of name → {description, ...}
+	if slotsRaw, ok := manifest["slots"]; ok {
+		slots, ok := slotsRaw.(map[string]any)
+		if !ok {
+			return "slots must be an object"
+		}
+		for name, v := range slots {
+			entry, ok := v.(map[string]any)
+			if !ok {
+				return "slots." + name + " must be an object"
+			}
+			if _, ok := entry["description"]; !ok {
+				return "slots." + name + " must have a description"
+			}
+		}
+	}
+
+	// Validate contributes: map of "pkg:slot" → {label, ...}
+	if contribRaw, ok := manifest["contributes"]; ok {
+		contribs, ok := contribRaw.(map[string]any)
+		if !ok {
+			return "contributes must be an object"
+		}
+		for key := range contribs {
+			if !strings.Contains(key, ":") {
+				return "contributes key " + key + " must follow pkg:slot convention"
+			}
+		}
+	}
+
+	return ""
+}
+
 // ── User endpoints ──────────────────────────────
 
 // ListUserExtensions returns all enabled extensions for the current user,
@@ -208,6 +246,12 @@ func (h *ExtensionHandler) AdminInstallExtension(c *gin.Context) {
 		manifestMap = map[string]any{}
 	}
 
+	// Validate composability fields (slots / contributes)
+	if errMsg := ValidateComposabilityFields(manifestMap); errMsg != "" {
+		c.JSON(400, gin.H{"error": "manifest: " + errMsg})
+		return
+	}
+
 	pkg := &store.PackageRegistration{
 		ID:          body.ExtID,
 		Title:       body.Name,