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/create-nextcloud-env

@@ -1,19 +1,19 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
-function create-nextcloud-env() {
-    if test -z "$1"; then echo "\$1 is empty."; exit; fi
+create-nextcloud-env() {
+    if [[ -z "$1" ]]; then echo "\$1 is empty."; exit; fi
 
     init-config-dir
-    ENV_FILEPATH="$TASK_CONFIG_DIR/.env.$1"
-    if [ -f "$ENV_FILEPATH" ]; then
-        echo "Env file already exists: $ENV_FILEPATH"
+    local env_filepath="$TASK_CONFIG_DIR/.env.$1"
+    if [[ -f "$env_filepath" ]]; then
+        echo "Env file already exists: $env_filepath"
         exit 1
     fi
 
-    echo "Init env file: $ENV_FILEPATH"
-    echo "NEXTCLOUD_URL=" > "$ENV_FILEPATH"
-    echo "NEXTCLOUD_USERNAME=" >> "$ENV_FILEPATH"
-    echo "NEXTCLOUD_PASSWORD=" >> "$ENV_FILEPATH"
+    echo "Init env file: $env_filepath"
+    echo "NEXTCLOUD_URL=" > "$env_filepath"
+    echo "NEXTCLOUD_USERNAME=" >> "$env_filepath"
+    echo "NEXTCLOUD_PASSWORD=" >> "$env_filepath"
 
     echo "Edit with: task edit-env $1"
 }