fix: run doctor version checks under fish

The doctor "can find npm/pi" checks wrap the version command in a POSIX
subshell, `(cmd --version 2>&1 || true)`. Fish treats `( ... )` as command
substitution syntax and forbids it in command position, so every fish user saw
false-negative failures:

  fish: command substitutions not allowed in command position
  command -v npm && (npm --version 2>&1 || true)

Branch on the detected service shell and emit fish's `begin; ...; end`
grouping for fish, mirroring the existing fish-aware quoting in
serviceShellQuote. Bash and zsh keep the POSIX subshell.

Also guard the `main()` invocation with an ESM main-module check so the CLI
helpers can be imported by tests without side effects, and add a regression
test covering the bash, zsh, and fish command shapes.

src/cli.test.ts

@@ -0,0 +1,31 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { commandWithVersionCheck } from "./cli.js";
+
+const originalShell = process.env["SHELL"];
+
+afterEach(() => {
+  if (originalShell === undefined) {
+    delete process.env["SHELL"];
+  } else {
+    process.env["SHELL"] = originalShell;
+  }
+});
+
+describe("commandWithVersionCheck", () => {
+  it("emits a POSIX subshell group for bash", () => {
+    process.env["SHELL"] = "/bin/bash";
+    expect(commandWithVersionCheck("npm")).toBe("command -v npm && (npm --version 2>&1 || true)");
+  });
+
+  it("emits a POSIX subshell group for zsh", () => {
+    process.env["SHELL"] = "/bin/zsh";
+    expect(commandWithVersionCheck("pi")).toBe("command -v pi && (pi --version 2>&1 || true)");
+  });
+
+  it("uses fish begin/end grouping instead of a POSIX subshell", () => {
+    process.env["SHELL"] = "/usr/local/bin/fish";
+    const command = commandWithVersionCheck("npm");
+    expect(command).toBe("command -v npm && begin; npm --version 2>&1 || true; end");
+    expect(command).not.toContain("(");
+  });
+});