feat(reactive): graph diagnostics and subagent_model config

Add getMissingAnnotationTasks() to surface which tasks lack IO
annotations and prevent parallel dispatch. Also add subagent_model
to ReactiveExecutionConfig for overriding the model used by
subagents during parallel task execution.

- getMissingAnnotationTasks() with 4 tests
- subagent_model field on ReactiveExecutionConfig type
- Validation for reactive_execution.subagent_model preference

src/resources/extensions/gsd/preferences-validation.ts

@@ -523,7 +523,15 @@ export function validatePreferences(preferences: GSDPreferences): {
         }
       }
 
-      const knownReKeys = new Set(["enabled", "max_parallel", "isolation_mode"]);
+      if (re.subagent_model !== undefined) {
+        if (typeof re.subagent_model === "string" && re.subagent_model.length > 0) {
+          validRe.subagent_model = re.subagent_model;
+        } else {
+          errors.push("reactive_execution.subagent_model must be a non-empty string");
+        }
+      }
+
+      const knownReKeys = new Set(["enabled", "max_parallel", "isolation_mode", "subagent_model"]);
       for (const key of Object.keys(re)) {
         if (!knownReKeys.has(key)) {
           warnings.push(`unknown reactive_execution key "${key}" — ignored`);

src/resources/extensions/gsd/reactive-graph.ts

@@ -131,6 +131,24 @@ export function isGraphAmbiguous(graph: DerivedTaskNode[]): boolean {
   );
 }
 
+/**
+ * Returns tasks that are missing IO annotations (no inputFiles and no outputFiles).
+ * These tasks prevent parallel dispatch by making the graph ambiguous.
+ * Used to surface actionable diagnostics when parallel execution falls back to sequential.
+ */
+export function getMissingAnnotationTasks(
+  graph: DerivedTaskNode[],
+): Array<{ id: string; title: string }> {
+  return graph
+    .filter(
+      (node) =>
+        !node.done &&
+        node.inputFiles.length === 0 &&
+        node.outputFiles.length === 0,
+    )
+    .map((node) => ({ id: node.id, title: node.title }));
+}
+
 /**
  * Detect deadlock: no tasks are ready and none are in-flight, yet incomplete
  * tasks remain. This indicates a circular dependency or impossible state.

src/resources/extensions/gsd/tests/reactive-graph.test.ts

@@ -5,6 +5,7 @@ import {
   getReadyTasks,
   chooseNonConflictingSubset,
   isGraphAmbiguous,
+  getMissingAnnotationTasks,
   detectDeadlock,
   graphMetrics,
 } from "../reactive-graph.ts";
@@ -297,3 +298,47 @@ test("graphMetrics computes correct values", () => {
   assert.equal(metrics.readySetSize, 2); // T02 (T01 done) and T03 (no deps)
   assert.equal(metrics.ambiguous, false);
 });
+
+// ─── getMissingAnnotationTasks ─────────────────────────────────────────────
+
+test("getMissingAnnotationTasks: returns empty array when all tasks have annotations", () => {
+  const graph: DerivedTaskNode[] = [
+    { id: "T01", title: "A", inputFiles: ["src/a.ts"], outputFiles: ["src/b.ts"], done: false, dependsOn: [] },
+    { id: "T02", title: "B", inputFiles: [], outputFiles: ["src/c.ts"], done: false, dependsOn: [] },
+  ];
+  assert.deepEqual(getMissingAnnotationTasks(graph), []);
+});
+
+test("getMissingAnnotationTasks: returns tasks with missing annotations", () => {
+  const graph: DerivedTaskNode[] = [
+    { id: "T01", title: "A", inputFiles: [], outputFiles: [], done: false, dependsOn: [] },
+    { id: "T02", title: "B", inputFiles: ["src/a.ts"], outputFiles: ["src/b.ts"], done: false, dependsOn: [] },
+    { id: "T03", title: "C", inputFiles: [], outputFiles: [], done: false, dependsOn: [] },
+  ];
+  assert.deepEqual(getMissingAnnotationTasks(graph), [
+    { id: "T01", title: "A" },
+    { id: "T03", title: "C" },
+  ]);
+});
+
+test("getMissingAnnotationTasks: skips done tasks", () => {
+  const graph: DerivedTaskNode[] = [
+    { id: "T01", title: "A", inputFiles: [], outputFiles: [], done: true, dependsOn: [] },
+    { id: "T02", title: "B", inputFiles: [], outputFiles: [], done: false, dependsOn: [] },
+  ];
+  assert.deepEqual(getMissingAnnotationTasks(graph), [
+    { id: "T02", title: "B" },
+  ]);
+});
+
+test("getMissingAnnotationTasks: returns only tasks missing BOTH inputFiles and outputFiles", () => {
+  const graph: DerivedTaskNode[] = [
+    { id: "T01", title: "InputOnly", inputFiles: ["src/a.ts"], outputFiles: [], done: false, dependsOn: [] },
+    { id: "T02", title: "OutputOnly", inputFiles: [], outputFiles: ["src/b.ts"], done: false, dependsOn: [] },
+    { id: "T03", title: "Neither", inputFiles: [], outputFiles: [], done: false, dependsOn: [] },
+    { id: "T04", title: "Both", inputFiles: ["src/c.ts"], outputFiles: ["src/d.ts"], done: false, dependsOn: [] },
+  ];
+  assert.deepEqual(getMissingAnnotationTasks(graph), [
+    { id: "T03", title: "Neither" },
+  ]);
+});

src/resources/extensions/gsd/types.ts

@@ -476,6 +476,8 @@ export interface ReactiveExecutionConfig {
   max_parallel: number;
   /** Isolation mode for parallel tasks within a slice. Currently only "same-tree" is supported. */
   isolation_mode: "same-tree";
+  /** Optional model override for subagents spawned during parallel execution. */
+  subagent_model?: string;
 }
 
 /** Per-slice reactive execution runtime state, persisted to disk. */