Use /usr/bin/env bash instead of /bin/bash for shebang

Some systems like NixOS don't have `/bin/bash`.
Ref: https://discourse.nixos.org/t/add-bin-bash-to-avoid-unnecessary-pain/5673

This changes allow to run scripts in those systems.

Fixes #43.

scripts/check-task-prerequisites.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Check that implementation plan exists and find optional design documents
 # Usage: ./check-task-prerequisites.sh [--json]
 

scripts/common.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Common functions and variables for all scripts
 
 # Get repository root

scripts/create-new-feature.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Create a new feature with branch, directory structure, and template
 # Usage: ./create-new-feature.sh "feature description"
 #        ./create-new-feature.sh --json "feature description"

scripts/get-feature-paths.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Get paths for current feature branch without creating anything
 # Used by commands that need to find existing feature files
 

scripts/setup-plan.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Setup implementation plan structure for current branch
 # Returns paths needed for implementation plan generation
 # Usage: ./setup-plan.sh [--json]

scripts/update-agent-context.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Incrementally update agent context files based on new feature plan
 # Supports: CLAUDE.md, GEMINI.md, and .github/copilot-instructions.md
 # O(1) operation - only reads current context file and new plan.md