feat(FN-2703): support custom MCP tools in plan generation

- Remove ToolSearch prerequisite so custom MCP tools can be used during triage and plan generation flows
- Align built-in tool sets between provider wiring and prompt builder handling, including custom ls behavior
- Expand pi-claude-cli tests for event bridge, MCP config, prompt builder, and tool mapping regressions
- Add FN-2703 changeset and delivery documentation for the published @runfusion/fusion package

packages/pi-claude-cli/src/__tests__/event-bridge.test.ts

@@ -970,6 +970,46 @@ describe("createEventBridge", () => {
       // Custom tool args pass through unchanged (no argument renames for MCP tools)
       expect(event.toolCall.arguments).toEqual({ target: "prod" });
     });
+
+    it("handles MCP-prefixed triage tool names", () => {
+      const bridge = createBridgeWithStart();
+      bridge.handleEvent({
+        type: "content_block_start",
+        index: 0,
+        content_block: {
+          type: "tool_use",
+          id: "toolu_triage1",
+          name: "mcp__custom-tools__fn_task_list",
+        },
+      });
+      bridge.handleEvent({
+        type: "content_block_stop",
+        index: 0,
+      });
+
+      bridge.handleEvent({
+        type: "content_block_start",
+        index: 1,
+        content_block: {
+          type: "tool_use",
+          id: "toolu_triage2",
+          name: "mcp__custom-tools__fn_review_spec",
+        },
+      });
+      bridge.handleEvent({
+        type: "content_block_stop",
+        index: 1,
+      });
+
+      const output = bridge.getOutput();
+      const calls = output.content.filter((c: any) => c.type === "toolCall") as any[];
+      expect(calls.map((call) => call.name)).toEqual([
+        "fn_task_list",
+        "fn_review_spec",
+      ]);
+      expect(calls[0].arguments).toEqual({});
+      expect(calls[1].arguments).toEqual({});
+    });
   });
 
   describe("internal Claude Code tools are filtered", () => {

packages/pi-claude-cli/src/__tests__/mcp-config.test.ts

@@ -16,7 +16,7 @@ vi.mock("node:os", () => ({
   tmpdir: mocks.tmpdir,
 }));
 
-import { getCustomToolDefs, writeMcpConfig } from "../mcp-config";
+import { getCustomToolDefs, writeMcpConfig, toolsFromContext } from "../mcp-config";
 import type { McpToolDef } from "../mcp-config";
 
 describe("getCustomToolDefs", () => {
@@ -171,6 +171,22 @@ describe("getCustomToolDefs", () => {
   });
 });
 
+describe("toolsFromContext", () => {
+  it("keeps ls as a custom tool (not filtered as built-in)", () => {
+    const defs = toolsFromContext([
+      { name: "read", description: "builtin", parameters: { type: "object" } },
+      { name: "ls", description: "list files", parameters: { type: "object" } },
+      {
+        name: "fn_task_list",
+        description: "list tasks",
+        parameters: { type: "object", properties: {} },
+      },
+    ]);
+
+    expect(defs.map((d) => d.name)).toEqual(["ls", "fn_task_list"]);
+  });
+});
+
 describe("writeMcpConfig", () => {
   beforeEach(() => {
     vi.clearAllMocks();

packages/pi-claude-cli/src/__tests__/prompt-builder.test.ts

@@ -949,6 +949,47 @@ describe("buildSystemPrompt", () => {
     expect(result).toContain("mcp__custom-tools__fn_review_spec");
   });
 
+  it("treats ls as custom and rewrites to mcp__custom-tools__ls", async () => {
+    vi.resetModules();
+    vi.doMock("node:fs", () => ({
+      existsSync: () => false,
+      readFileSync: () => "",
+    }));
+
+    const { buildSystemPrompt: bsp } = await import("../prompt-builder");
+    const context = {
+      systemPrompt: "List files with ls, then continue.",
+      messages: [],
+      tools: [
+        { name: "ls", description: "list directory", parameters: {} },
+        { name: "read", description: "builtin", parameters: {} },
+      ],
+    } as unknown as any;
+
+    const result = bsp(context, "/some/project");
+    expect(result).toContain("mcp__custom-tools__ls");
+    expect(result).not.toContain("mcp__custom-tools__read");
+  });
+
+  it("custom tools addendum instructs direct MCP calls without ToolSearch prerequisite", async () => {
+    vi.resetModules();
+    vi.doMock("node:fs", () => ({
+      existsSync: () => false,
+      readFileSync: () => "",
+    }));
+
+    const { buildSystemPrompt: bsp } = await import("../prompt-builder");
+    const context = {
+      systemPrompt: "Use tools as needed.",
+      messages: [],
+      tools: [{ name: "fn_task_list", description: "list", parameters: {} }],
+    } as unknown as any;
+
+    const result = bsp(context, "/some/project");
+    expect(result).toContain("mcp__custom-tools__fn_task_list");
+    expect(result).not.toContain("ToolSearch");
+  });
+
   it("does not rewrite identifier substrings that happen to overlap a tool name", async () => {
     vi.resetModules();
     vi.doMock("node:fs", () => ({

packages/pi-claude-cli/src/__tests__/tool-mapping.test.ts

@@ -228,6 +228,7 @@ describe("tool-mapping", () => {
     it("returns true for custom tool names", () => {
       expect(isCustomToolName("myTool")).toBe(true);
       expect(isCustomToolName("deploy")).toBe(true);
+      expect(isCustomToolName("ls")).toBe(true);
     });
 
     it("returns false for all 6 built-in tool names", () => {