feat(FN-3140): add readonly custom tools and plugin SDK

This merge adds readonly custom tool preservation (FN-3140) with new plugin SDK types and documentation, fixes PluginManager responsive overflow (FN-3093), and integrates the fn-3065 branch with enhanced plugin authoring capabilities. The core plugin-types module was significantly expanded with 230+

Fusion-Task-Id: FN-3140

.changeset/fn-3140-fix-readonly-customtools.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix readonly `createFnAgent` sessions to preserve caller-supplied engine custom tools while still excluding host extensions. This restores delegation and memory tools for no-task heartbeat/reviewer readonly sessions without reopening host extension tool injection in summarizer flows.

packages/engine/src/__tests__/pi-create-fn-agent.test.ts

@@ -594,6 +594,50 @@ describe("createFnAgent", () => {
     });
   });
 
+  it("keeps caller customTools in readonly sessions", async () => {
+    createReadOnlyToolsMock.mockReturnValueOnce([{ name: "read" }] as any);
+    const delegationTool = {
+      name: "fn_list_agents",
+      label: "List Agents",
+      description: "List available agents",
+      parameters: {},
+      execute: vi.fn(),
+    };
+
+    const { createFnAgent } = await import("../pi.js");
+    await createFnAgent({
+      cwd: "/tmp",
+      systemPrompt: "test",
+      tools: "readonly",
+      customTools: [delegationTool as any],
+    });
+
+    const createSessionArgs = createAgentSessionMock.mock.calls[0]?.[0] as { customTools: Array<{ name: string }> };
+    expect(createSessionArgs.customTools.map((tool) => tool.name)).toContain("fn_list_agents");
+  });
+
+  it("keeps caller customTools in coding sessions", async () => {
+    createCodingToolsMock.mockReturnValueOnce([{ name: "read" }, { name: "write" }] as any);
+    const customTool = {
+      name: "fn_heartbeat_done",
+      label: "Heartbeat Done",
+      description: "Complete heartbeat",
+      parameters: {},
+      execute: vi.fn(),
+    };
+
+    const { createFnAgent } = await import("../pi.js");
+    await createFnAgent({
+      cwd: "/tmp",
+      systemPrompt: "test",
+      tools: "coding",
+      customTools: [customTool as any],
+    });
+
+    const createSessionArgs = createAgentSessionMock.mock.calls[0]?.[0] as { customTools: Array<{ name: string }> };
+    expect(createSessionArgs.customTools.map((tool) => tool.name)).toContain("fn_heartbeat_done");
+  });
+
   it("logs createFnAgent startup diagnostics without leaking cwd", async () => {
     const { piLog } = await import("../logger.js");
     const logSpy = vi.spyOn(piLog, "log").mockImplementation(() => {});

packages/engine/src/pi.ts

@@ -1146,12 +1146,13 @@ export async function createFnAgent(options: AgentOptions): Promise<AgentResult>
     });
   }
 
-  // `tools: "readonly"` MUST mean a hermetically sealed read-only session — no
+  // `tools: "readonly"` MUST mean a hermetically sealed read-only session with
-  // way for the model to mutate state. Host extensions (`@runfusion/fusion`)
+  // respect to host extension injection. Host extensions (`@runfusion/fusion`)
-  // register write tools like `fn_task_create`, so they are deliberately
+  // can register write tools like `fn_task_create`, so they are deliberately
-  // EXCLUDED in readonly mode. Caller-supplied `customTools` are also dropped
+  // EXCLUDED in readonly mode. Caller-supplied `customTools` are preserved,
-  // for the same reason. Without this, summarizer/compaction sessions could
+  // since heartbeat/reviewer flows explicitly provide engine-owned tools.
-  // call write tools and mutate the task board (see FN-3057/FN-3058 incident).
+  // This keeps summarizer/compaction sessions safe while retaining intended
+  // delegation/memory tools for readonly engine sessions.
   const isReadonly = options.tools === "readonly";
   const effectiveExtensionPaths = isReadonly ? [] : hostExtensionPaths;
   if (isReadonly && hostExtensionPaths.length > 0) {
@@ -1178,15 +1179,10 @@ export async function createFnAgent(options: AgentOptions): Promise<AgentResult>
     // suppress the defaults with `noTools: "builtin"` and register our wrapped
     // tools through `customTools` instead. The wrapped tools preserve the same
     // names (`read`, `bash`, ...) as the built-ins they replace.
-    // Readonly sessions drop caller-supplied customTools — see comment above
-    // about hermetic isolation. Only the wrapped read-only built-ins survive.
     const customToolList: ToolDefinition[] = [
       ...(wrappedTools as ToolDefinition[]),
-      ...(isReadonly ? [] : (options.customTools ?? [])),
+      ...(options.customTools ?? []),
     ];
-    if (isReadonly && (options.customTools?.length ?? 0) > 0) {
-      piLog.log(`readonly session — customTools (${options.customTools!.length}) skipped`);
-    }
     // Last-chance abort hook. Fires *here* — after every awaited setup step
     // in createFnAgent (provider registration, worktree validation, resource
     // loader reload) and immediately before the actual LLM session spawn.