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/packages.go

@@ -212,10 +212,48 @@ func (h *PackageHandler) DeletePackage(c *gin.Context) {
 		}
 	}
 
-	c.JSON(http.StatusOK, gin.H{"id": id, "deleted": true})
+	resp := gin.H{"id": id, "deleted": true}
+
+	// Warn about orphaned slot contributions
+	if orphaned := findOrphanedContributors(c, h.stores, id); len(orphaned) > 0 {
+		resp["warning"] = "slot contributions orphaned in: " + strings.Join(orphaned, ", ")
+	}
+
+	c.JSON(http.StatusOK, resp)
 	h.broadcastPackageChanged("deleted", id)
 }
 
+// findOrphanedContributors returns package IDs that declare contributes
+// targeting slots owned by the given package (e.g. "notes:toolbar-actions"
+// targets the "notes" package). This is a warning, not a blocker.
+func findOrphanedContributors(c *gin.Context, stores store.Stores, deletedPkgID string) []string {
+	allPkgs, err := stores.Packages.List(c.Request.Context())
+	if err != nil {
+		return nil
+	}
+	prefix := deletedPkgID + ":"
+	seen := map[string]bool{}
+	for _, p := range allPkgs {
+		if p.ID == deletedPkgID || !p.Enabled {
+			continue
+		}
+		contribs, ok := p.Manifest["contributes"].(map[string]any)
+		if !ok {
+			continue
+		}
+		for key := range contribs {
+			if strings.HasPrefix(key, prefix) {
+				seen[p.ID] = true
+			}
+		}
+	}
+	result := make([]string, 0, len(seen))
+	for id := range seen {
+		result = append(result, id)
+	}
+	return result
+}
+
 // InstallPackage uploads and installs a .pkg/.surface archive.
 // POST /api/v1/admin/packages/install
 //
@@ -466,6 +504,13 @@ func (h *PackageHandler) parseAndValidateArchive(c *gin.Context, tmpPath string)
 		return nil, nil, nil, err
 	}
 
+	// Validate composability fields (slots / contributes)
+	if errMsg := ValidateComposabilityFields(manifest); errMsg != "" {
+		zr.Close()
+		c.JSON(http.StatusBadRequest, gin.H{"error": "manifest: " + errMsg})
+		return nil, nil, nil, fmt.Errorf("invalid composability fields")
+	}
+
 	// Signing hook (reserved)
 	if os.Getenv("PACKAGE_VERIFY_SIGNATURES") == "true" {
 		if mInfo.Signature != "" {