feat(coding-agent): add --no-tools flag to disable built-in tools

Add --no-tools flag that allows starting pi without any built-in tools,
enabling extension-only tool setups (e.g., pi-ssh-remote).

- Add --no-tools flag to CLI args parsing
- Handle --tools '' (empty string) as equivalent to no tools
- Fix system prompt to not show READ-ONLY mode when no tools (extensions may provide write capabilities)
- Add tests for new flag and system prompt behavior

fixes #555

packages/coding-agent/test/args.test.ts

@@ -170,6 +170,24 @@ describe("parseArgs", () => {
 		});
 	});
 
+	describe("--no-tools flag", () => {
+		test("parses --no-tools flag", () => {
+			const result = parseArgs(["--no-tools"]);
+			expect(result.noTools).toBe(true);
+		});
+
+		test("parses --no-tools with explicit --tools flags", () => {
+			const result = parseArgs(["--no-tools", "--tools", "read,bash"]);
+			expect(result.noTools).toBe(true);
+			expect(result.tools).toEqual(["read", "bash"]);
+		});
+
+		test("parses --tools with empty string", () => {
+			const result = parseArgs(["--tools", ""]);
+			expect(result.tools).toEqual([]);
+		});
+	});
+
 	describe("messages and file args", () => {
 		test("parses plain text messages", () => {
 			const result = parseArgs(["hello", "world"]);

packages/coding-agent/test/system-prompt.test.ts

@@ -0,0 +1,65 @@
+import { describe, expect, test } from "vitest";
+import { buildSystemPrompt } from "../src/core/system-prompt.js";
+
+describe("buildSystemPrompt", () => {
+	describe("empty tools", () => {
+		test("does not show READ-ONLY mode when no built-in tools", () => {
+			const prompt = buildSystemPrompt({
+				selectedTools: [],
+				contextFiles: [],
+				skills: [],
+			});
+
+			// Should not mention READ-ONLY mode when there are no tools
+			// (extensions may provide write capabilities)
+			expect(prompt).not.toContain("READ-ONLY mode");
+		});
+
+		test("shows (none) for empty tools list", () => {
+			const prompt = buildSystemPrompt({
+				selectedTools: [],
+				contextFiles: [],
+				skills: [],
+			});
+
+			expect(prompt).toContain("Available tools:\n(none)");
+		});
+
+		test("does not show file paths guideline when no tools", () => {
+			const prompt = buildSystemPrompt({
+				selectedTools: [],
+				contextFiles: [],
+				skills: [],
+			});
+
+			expect(prompt).not.toContain("Show file paths clearly");
+		});
+	});
+
+	describe("read-only tools", () => {
+		test("shows READ-ONLY mode when only read tools available", () => {
+			const prompt = buildSystemPrompt({
+				selectedTools: ["read", "grep", "find", "ls"],
+				contextFiles: [],
+				skills: [],
+			});
+
+			expect(prompt).toContain("READ-ONLY mode");
+		});
+	});
+
+	describe("default tools", () => {
+		test("does not show READ-ONLY mode with default tools", () => {
+			const prompt = buildSystemPrompt({
+				contextFiles: [],
+				skills: [],
+			});
+
+			expect(prompt).not.toContain("READ-ONLY mode");
+			expect(prompt).toContain("- read:");
+			expect(prompt).toContain("- bash:");
+			expect(prompt).toContain("- edit:");
+			expect(prompt).toContain("- write:");
+		});
+	});
+});