feat(FN-1772): add backend-aware memory instruction context API

- Add MemoryBackendContext interface for backend-aware memory integration
- Implement getMemoryInstructions() method with backend-specific logic in project-memory.ts
- Add QMD memory backend type detection and instruction context for QMD-backed projects
- Update memory-plugin-contract.md with backend-variant instruction requirements (section 3.8.7)
- Update settings reference for memoryBackendType with custom backend support
- Add comprehensive regression tests for backend-variant memory instruction behavior
- Add changeset for @gsxdsm/fusion package

packages/engine/src/executor.test.ts

@@ -2357,6 +2357,65 @@ describe("buildExecutionPrompt", () => {
       expect(result).toContain("`.fusion/memory.md`");
     });
   });
+
+  describe("memoryBackendType setting", () => {
+    it("includes .fusion/memory.md for file backend", () => {
+      const task = createMockTaskDetail();
+      const result = buildExecutionPrompt(task, "/project", {
+        memoryEnabled: true,
+        memoryBackendType: "file",
+      } as any);
+      expect(result).toContain("## Project Memory");
+      // Check that the Project Memory section contains .fusion/memory.md
+      const memorySectionMatch = result.match(/## Project Memory\n([\s\S]*?)(?=\n## [^#]|$)/);
+      expect(memorySectionMatch).toBeTruthy();
+      expect(memorySectionMatch![1]).toContain(".fusion/memory.md");
+    });
+
+    it("includes read-only wording for readonly backend without write directives in memory section", () => {
+      const task = createMockTaskDetail();
+      const result = buildExecutionPrompt(task, "/project", {
+        memoryEnabled: true,
+        memoryBackendType: "readonly",
+      } as any);
+      expect(result).toContain("## Project Memory");
+      // Extract the Project Memory section
+      const memorySectionMatch = result.match(/## Project Memory\n([\s\S]*?)(?=\n## [^#]|$)/);
+      expect(memorySectionMatch).toBeTruthy();
+      const memorySection = memorySectionMatch![1];
+      // Should NOT contain write/update directives in the memory section
+      expect(memorySection).not.toMatch(/write.*memory|update.*memory/i);
+      // Should NOT contain the specific file path in the memory section
+      expect(memorySection).not.toContain(".fusion/memory.md");
+    });
+
+    it("does not include .fusion/memory.md in Project Memory section for qmd backend", () => {
+      const task = createMockTaskDetail();
+      const result = buildExecutionPrompt(task, "/project", {
+        memoryEnabled: true,
+        memoryBackendType: "qmd",
+      } as any);
+      expect(result).toContain("## Project Memory");
+      // Extract the Project Memory section
+      const memorySectionMatch = result.match(/## Project Memory\n([\s\S]*?)(?=\n## [^#]|$)/);
+      expect(memorySectionMatch).toBeTruthy();
+      const memorySection = memorySectionMatch![1];
+      // QMD should NOT unconditionally reference .fusion/memory.md in the memory section
+      expect(memorySection).not.toContain(".fusion/memory.md");
+      // Should instruct to consult project memory
+      expect(memorySection).toMatch(/consult.*project memory/i);
+    });
+
+    it("excludes memory section when memoryEnabled: false regardless of backend", () => {
+      const task = createMockTaskDetail();
+      const result = buildExecutionPrompt(task, "/project", {
+        memoryEnabled: false,
+        memoryBackendType: "file",
+      } as any);
+      expect(result).toContain("Execute this task.");
+      expect(result).not.toContain("## Project Memory");
+    });
+  });
 });
 
 // Import the summarizeToolArgs helper directly (not affected by mocks above)

packages/engine/src/executor.ts

@@ -4049,11 +4049,12 @@ git log --oneline
   }
 
   // Build project memory section from settings
-  // When enabled, agents consult and update .fusion/memory.md for durable project learnings.
+  // When enabled, agents consult and update project memory for durable project learnings.
+  // Backend-aware: instructions branch based on memoryBackendType (file, readonly, qmd)
   const memoryEnabled = settings?.memoryEnabled !== false;
   let memorySection = "";
   if (memoryEnabled && rootDir) {
-    memorySection = "\n" + buildExecutionMemoryInstructions(rootDir);
+    memorySection = "\n" + buildExecutionMemoryInstructions(rootDir, settings);
   }
 
   // Build steering comments section (last 10 comments only to avoid context bloat)

packages/engine/src/triage.test.ts

@@ -317,6 +317,71 @@ describe("buildSpecificationPrompt", () => {
     });
   });
 
+  describe("memoryBackendType setting", () => {
+    it("includes .fusion/memory.md for file backend", () => {
+      const settings: Settings = {
+        maxConcurrent: 2,
+        maxWorktrees: 4,
+        pollIntervalMs: 10000,
+        groupOverlappingFiles: false,
+        autoMerge: true,
+        memoryEnabled: true,
+        memoryBackendType: "file",
+      };
+      const prompt = buildSpecificationPrompt(
+        baseTask,
+        ".fusion/tasks/KB-001/PROMPT.md",
+        settings,
+      );
+      expect(prompt).toContain("## Project Memory");
+      expect(prompt).toContain(".fusion/memory.md");
+    });
+
+    it("includes read-only wording for readonly backend without write directives", () => {
+      const settings: Settings = {
+        maxConcurrent: 2,
+        maxWorktrees: 4,
+        pollIntervalMs: 10000,
+        groupOverlappingFiles: false,
+        autoMerge: true,
+        memoryEnabled: true,
+        memoryBackendType: "readonly",
+      };
+      const prompt = buildSpecificationPrompt(
+        baseTask,
+        ".fusion/tasks/KB-001/PROMPT.md",
+        settings,
+      );
+      expect(prompt).toContain("## Project Memory");
+      // Should NOT contain write/update directives
+      expect(prompt).not.toMatch(/write.*memory|update.*memory/i);
+      // Should NOT contain the specific file path
+      expect(prompt).not.toContain(".fusion/memory.md");
+    });
+
+    it("does not include .fusion/memory.md for qmd backend", () => {
+      const settings: Settings = {
+        maxConcurrent: 2,
+        maxWorktrees: 4,
+        pollIntervalMs: 10000,
+        groupOverlappingFiles: false,
+        autoMerge: true,
+        memoryEnabled: true,
+        memoryBackendType: "qmd",
+      };
+      const prompt = buildSpecificationPrompt(
+        baseTask,
+        ".fusion/tasks/KB-001/PROMPT.md",
+        settings,
+      );
+      expect(prompt).toContain("## Project Memory");
+      // QMD should NOT unconditionally reference .fusion/memory.md
+      expect(prompt).not.toContain(".fusion/memory.md");
+      // Should instruct to consult project memory
+      expect(prompt).toMatch(/consult.*project memory/i);
+    });
+  });
+
   describe("user comments", () => {
     it("includes user comments section when user comments exist", () => {
       const taskWithComments: TaskDetail = {

packages/engine/src/triage.ts

@@ -1410,11 +1410,12 @@ export function buildSpecificationPrompt(
   }
 
   // Build project memory section from settings.
-  // When enabled, agents consult .fusion/memory.md for durable project learnings.
+  // When enabled, agents consult project memory for durable project learnings.
+  // Backend-aware: instructions branch based on memoryBackendType (file, readonly, qmd)
   const memoryEnabled = settings?.memoryEnabled !== false;
   let memorySection = "";
   if (memoryEnabled) {
-    memorySection = "\n\n" + buildTriageMemoryInstructions("");
+    memorySection = "\n\n" + buildTriageMemoryInstructions("", settings);
   }
 
   let attachmentsSection = "";