docs(agents): refine bash guidelines and update code samples

- Expand AGENTS.md with general agent guidance and coding conventions
- Refactor bash snippets in README.md and scripts to align with style guide:
  - Use `#!/usr/bin/env bash` shebang for portability
  - Remove `function` keyword from definitions
  - Replace `[ ]` conditionals with `[[ ]]` for improved reliability
  - Rename local variables to lowercase with `local` keyword
- Introduce CHANGELOG.md to track project changes per Keep a Changelog spec
- Update taskfile.build scripts to follow consistent bash practices

image/entrypoint.sh

@@ -1,17 +1,16 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -e
 
 echo "pnpm version: $(pnpm --version)"
 echo "hugo version: $(hugo version)"
 
-
-if [ -z "$GIT_URL" ]; then
+if [[ -z "$GIT_URL" ]]; then
     echo "Error: GIT_URL environment variable is required." >&2
     exit 1
 fi
 
 GIT_PATH="/app/repo"
-if [ ! -d "$GIT_PATH" ]; then
+if [[ ! -d "$GIT_PATH" ]]; then
     echo "Cloning git repo: $GIT_URL"
     git clone "$GIT_URL" "$GIT_PATH"
 else
@@ -19,15 +18,15 @@ else
     git -C "$GIT_PATH" pull
 fi
 
-if [ -f "$GIT_PATH/.gitmodules" ]; then
+if [[ -f "$GIT_PATH/.gitmodules" ]]; then
     echo "Found .gitmodules, initializing submodules."
     git -C "$GIT_PATH" submodule init
     git -C "$GIT_PATH" submodule update --recursive
 fi
 
-if [ -f "${GIT_PATH}/task" ]; then
+if [[ -f "${GIT_PATH}/task" ]]; then
 
-    if [ -z "$TASK_COMMANDS" ]; then
+    if [[ -z "$TASK_COMMANDS" ]]; then
         echo "Error: TASK_COMMANDS environment variable is required." >&2
         exit 1
     fi