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/update-submodule

@@ -1,6 +1,6 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
-function update-submodule() {
+update-submodule() {
     echo "Update git submodules."
     git submodule update --init --recursive --checkout --depth=1
 }