feat(coding-agent): add --no-extensions flag to disable extension discovery

Adds --no-extensions CLI flag that skips automatic extension discovery
while still allowing explicit -e paths. Three modes now available:

1. Default: auto-discover + any -e additions
2. --no-extensions: no extensions at all
3. --no-extensions -e foo.js: only load explicit extensions

Useful for debugging or running subagent instances without auto-discovered
extensions.

closes #524

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

@@ -150,6 +150,26 @@ describe("parseArgs", () => {
 		});
 	});
 
+	describe("--no-extensions flag", () => {
+		test("parses --no-extensions flag", () => {
+			const result = parseArgs(["--no-extensions"]);
+			expect(result.noExtensions).toBe(true);
+		});
+
+		test("parses --no-extensions with explicit -e flags", () => {
+			const result = parseArgs(["--no-extensions", "-e", "foo.ts", "-e", "bar.ts"]);
+			expect(result.noExtensions).toBe(true);
+			expect(result.extensions).toEqual(["foo.ts", "bar.ts"]);
+		});
+	});
+
+	describe("--no-skills flag", () => {
+		test("parses --no-skills flag", () => {
+			const result = parseArgs(["--no-skills"]);
+			expect(result.noSkills).toBe(true);
+		});
+	});
+
 	describe("messages and file args", () => {
 		test("parses plain text messages", () => {
 			const result = parseArgs(["hello", "world"]);

packages/coding-agent/test/extensions-discovery.test.ts

@@ -443,4 +443,34 @@ describe("extensions discovery", () => {
 		expect(result.extensions).toHaveLength(1);
 		expect(result.extensions[0].flags.has("--my-flag")).toBe(true);
 	});
+
+	it("loadExtensions only loads explicit paths without discovery", async () => {
+		// Create discoverable extensions (would be found by discoverAndLoadExtensions)
+		fs.writeFileSync(path.join(extensionsDir, "discovered.ts"), extensionCodeWithTool("discovered"));
+
+		// Create explicit extension outside discovery path
+		const explicitPath = path.join(tempDir, "explicit.ts");
+		fs.writeFileSync(explicitPath, extensionCodeWithTool("explicit"));
+
+		// Use loadExtensions directly to skip discovery
+		const { loadExtensions } = await import("../src/core/extensions/loader.js");
+		const result = await loadExtensions([explicitPath], tempDir);
+
+		expect(result.errors).toHaveLength(0);
+		expect(result.extensions).toHaveLength(1);
+		expect(result.extensions[0].tools.has("explicit")).toBe(true);
+		expect(result.extensions[0].tools.has("discovered")).toBe(false);
+	});
+
+	it("loadExtensions with no paths loads nothing", async () => {
+		// Create discoverable extensions (would be found by discoverAndLoadExtensions)
+		fs.writeFileSync(path.join(extensionsDir, "discovered.ts"), extensionCode);
+
+		// Use loadExtensions directly with empty paths
+		const { loadExtensions } = await import("../src/core/extensions/loader.js");
+		const result = await loadExtensions([], tempDir);
+
+		expect(result.errors).toHaveLength(0);
+		expect(result.extensions).toHaveLength(0);
+	});
 });