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

bin/edit-env

@@ -1,8 +1,8 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
-function edit-env() {
-    if test -z "$1"; then echo "\$1 is empty."; exit; fi
+edit-env() {
+    if [[ -z "$1" ]]; then echo "\$1 is empty."; exit; fi
     TASK_CONFIG_DIR=${TASK_CONFIG_DIR:="$HOME/.config/taskfile.build"}
-    ENV_FILEPATH="$TASK_CONFIG_DIR/.env.$1"
-    "$EDITOR" "$ENV_FILEPATH"
+    local env_filepath="$TASK_CONFIG_DIR/.env.$1"
+    "$EDITOR" "$env_filepath"
 }