fix(org): per-workspace plugins UNION with defaults; '!' prefix opts out (#68)

Per-workspace `plugins:` now UNIONS with `defaults.plugins` instead of
replacing. A leading `!` or `-` on a per-workspace entry opts a default
out. Backward-compatible: re-listing defaults still dedupes to the same
list.

Refactored the inline REPLACE logic into a pure helper `mergePlugins`
in org.go so it's unit-testable. Five TestPlugins_* cases added.

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

platform/internal/handlers/org.go

@@ -340,11 +340,10 @@ func (h *OrgHandler) createWorkspaceTree(ws OrgWorkspace, parentID *string, defa
 			}
 		}
 
-		// Pre-install plugins: copy from registry into configFiles as plugins/<name>/*
-		plugins := ws.Plugins
-		if len(plugins) == 0 {
-			plugins = defaults.Plugins
-		}
+		// Pre-install plugins: copy from registry into configFiles as plugins/<name>/*.
+		// Per-workspace plugins UNION with defaults.plugins (issue #68).
+		// A leading "!" or "-" on a per-workspace entry opts that plugin out.
+		plugins := mergePlugins(defaults.Plugins, ws.Plugins)
 		if len(plugins) > 0 {
 			if configFiles == nil {
 				configFiles = map[string][]byte{}
@@ -642,3 +641,45 @@ func parseEnvFile(path string, out map[string]string) {
 		}
 	}
 }
+
+// mergePlugins returns the union of defaults and per-workspace plugin lists
+// (deduplicated, defaults first). A per-workspace entry starting with "!" or
+// "-" opts that plugin OUT of the union. See issue #68.
+func mergePlugins(defaultPlugins, wsPlugins []string) []string {
+	seen := map[string]bool{}
+	out := make([]string, 0, len(defaultPlugins)+len(wsPlugins))
+	for _, p := range defaultPlugins {
+		if p == "" || seen[p] {
+			continue
+		}
+		seen[p] = true
+		out = append(out, p)
+	}
+	for _, p := range wsPlugins {
+		if p == "" {
+			continue
+		}
+		if strings.HasPrefix(p, "!") || strings.HasPrefix(p, "-") {
+			target := strings.TrimLeft(p, "!-")
+			if target == "" {
+				continue
+			}
+			if seen[target] {
+				delete(seen, target)
+				filtered := out[:0]
+				for _, existing := range out {
+					if existing != target {
+						filtered = append(filtered, existing)
+					}
+				}
+				out = filtered
+			}
+			continue
+		}
+		if !seen[p] {
+			seen[p] = true
+			out = append(out, p)
+		}
+	}
+	return out
+}

platform/internal/handlers/org_test.go

@@ -383,3 +383,57 @@ func TestHasUnresolvedVarRef_DollarVarSyntax(t *testing.T) {
 		t.Error("$VAR syntax should be detected as ref when unresolved")
 	}
 }
+
+func eqStringSlice(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func TestPlugins_UnionWithDefaults(t *testing.T) {
+	got := mergePlugins([]string{"a", "b"}, []string{"c"})
+	want := []string{"a", "b", "c"}
+	if !eqStringSlice(got, want) {
+		t.Fatalf("got %v, want %v", got, want)
+	}
+}
+
+func TestPlugins_DedupesDuplicates(t *testing.T) {
+	got := mergePlugins([]string{"a", "b"}, []string{"b", "c"})
+	want := []string{"a", "b", "c"}
+	if !eqStringSlice(got, want) {
+		t.Fatalf("got %v, want %v", got, want)
+	}
+}
+
+func TestPlugins_OptOutWithBang(t *testing.T) {
+	got := mergePlugins([]string{"a", "b", "c"}, []string{"!b", "d"})
+	want := []string{"a", "c", "d"}
+	if !eqStringSlice(got, want) {
+		t.Fatalf("got %v, want %v", got, want)
+	}
+}
+
+func TestPlugins_OptOutWithDash(t *testing.T) {
+	got := mergePlugins([]string{"a", "b"}, []string{"-a"})
+	want := []string{"b"}
+	if !eqStringSlice(got, want) {
+		t.Fatalf("got %v, want %v", got, want)
+	}
+}
+
+func TestPlugins_BackwardCompat(t *testing.T) {
+	// Re-listing defaults in per-workspace plugins still yields the same list
+	// (dedupe keeps behavior stable for existing org.yaml files).
+	got := mergePlugins([]string{"a", "b"}, []string{"a", "b", "c"})
+	want := []string{"a", "b", "c"}
+	if !eqStringSlice(got, want) {
+		t.Fatalf("got %v, want %v", got, want)
+	}
+}