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/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