feat(FN-3097): wire plugin prompt contributions into execution surfaces

- Add plugin prompt contribution support across executor, triage, reviewer, and heartbeat instruction builders
- Thread plugin runner context into execution prompt assembly and preserve source issue commit reference hints
- Expand engine tests to cover prompt contribution injection behavior and regression scenarios
- Update architecture and plugin authoring docs with prompt surface integration details

Fusion-Task-Id: FN-3097

packages/engine/src/__tests__/agent-instructions.test.ts

@@ -8,6 +8,7 @@ import {
   resolveAgentInstructionsWithRatings,
   buildAgentChatPrompt,
   buildSystemPromptWithInstructions,
+  buildPluginPromptSection,
   ensureDefaultHeartbeatProcedureFile,
   resolveAgentHeartbeatProcedure,
 } from "../agent-instructions.js";
@@ -555,6 +556,38 @@ describe("buildSystemPromptWithInstructions", () => {
   });
 });
 
+describe("buildPluginPromptSection", () => {
+  it("returns empty string when pluginRunner is undefined", () => {
+    expect(buildPluginPromptSection("triage", undefined)).toBe("");
+  });
+
+  it("returns empty string when no contributions match", () => {
+    const pluginRunner = {
+      getPromptContributionsForSurface: vi.fn().mockReturnValue([]),
+    };
+
+    expect(buildPluginPromptSection("triage", pluginRunner as any)).toBe("");
+  });
+
+  it("formats grouped plugin sections and prepend-before-append ordering", () => {
+    const pluginRunner = {
+      getPromptContributionsForSurface: vi.fn().mockReturnValue([
+        { pluginId: "plugin-b", contribution: { surface: "triage", content: "append B1" }, config: {} },
+        { pluginId: "plugin-a", contribution: { surface: "triage", content: "prepend A1", position: "prepend" }, config: {} },
+        { pluginId: "plugin-a", contribution: { surface: "triage", content: "prepend A2", position: "prepend" }, config: {} },
+        { pluginId: "plugin-c", contribution: { surface: "triage", content: "append C1", position: "append" }, config: {} },
+      ]),
+    };
+
+    const result = buildPluginPromptSection("triage", pluginRunner as any);
+
+    expect(result).toContain("## Plugin: plugin-a\n\nprepend A1\n\nprepend A2");
+    expect(result).toContain("## Plugin: plugin-b\n\nappend B1");
+    expect(result).toContain("## Plugin: plugin-c\n\nappend C1");
+    expect(result.indexOf("## Plugin: plugin-a")).toBeLessThan(result.indexOf("## Plugin: plugin-b"));
+  });
+});
+
 describe("diagnostics logging", () => {
   let testDir: string;