🤖 fix: replace /bin/bash with /usr/bin/env bash for portability (#603)

Replace hardcoded `/bin/bash` shebangs with `/usr/bin/env bash` for
better portability across different systems, particularly NixOS where
`/bin/bash` does not exist.

## Changes

- **Makefile**: Update SHELL variable for non-Windows systems
- **docs/init-hooks.md**: Update example script shebangs (2 occurrences)
- **scripts/setup-macos-signing.sh**: Update shebang
- **vscode/scripts/create-icon.sh**: Update shebang

## Why

`/usr/bin/env bash` resolves bash from PATH, making scripts work on
systems where bash is not in `/bin` (e.g., NixOS installs bash in
`/nix/store/...`). This is a standard practice for portable shell
scripts.

_Generated with `mux`_

Author: kylecarbs

Makefile

@@ -24,12 +24,14 @@
 #   Branches reduce reproducibility - builds should fail fast with clear errors
 #   if dependencies are missing, not silently fall back to different behavior.
 
-# Use PATH-resolved bash on Windows to avoid hardcoded /usr/bin/bash which doesn't
-# exist in Chocolatey's make environment or on GitHub Actions windows-latest.
+# Use PATH-resolved bash for portability across different systems.
+# - Windows: /usr/bin/bash doesn't exist in Chocolatey's make environment or GitHub Actions
+# - NixOS: /bin/bash doesn't exist, bash is in /nix/store/...
+# - Other systems: /usr/bin/env bash resolves from PATH
 ifeq ($(OS),Windows_NT)
 SHELL := bash
 else
-SHELL := /bin/bash
+SHELL := /usr/bin/env bash
 endif
 .SHELLFLAGS := -eu -o pipefail -c
 

docs/init-hooks.md

@@ -5,7 +5,7 @@ Add a `.mux/init` executable script to your project root to run commands when cr
 ## Example
 
 ```bash
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 bun install
@@ -40,7 +40,7 @@ Init hooks receive the following environment variables:
 **Note for SSH workspaces:** Since the project is synced to the remote machine, files exist in both locations. The init hook runs in the workspace directory (`$PWD`), so use relative paths to reference project files:
 
 ```bash
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 echo "Runtime: $MUX_RUNTIME"

scripts/setup-macos-signing.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Sets up macOS code signing and notarization from GitHub secrets
 # Usage: ./scripts/setup-macos-signing.sh
 #

vscode/scripts/create-icon.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Create icon.png from the cmux logo for the VS Code extension
 
 set -e