Add support for XDG spec

Hi. At some point, cagent became part of Docker Desktop. Now, when I open Docker Desktop, cagent creates its own config directory. The problem is that cagent creates two of them: one in my home directory, another in .config/. Here is the log:

~
$ env | grep "XDG_"                                                     20:21:54
XDG_CACHE_HOME=/Users/vanyauhalin/Library/Caches
XDG_CONFIG_HOME=/Users/vanyauhalin/.config
XDG_DATA_DIRS=/usr/local/share:/usr/share:/Applications/Ghostty.app/Contents/Resources/ghostty/..
XDG_DATA_HOME=/Users/vanyauhalin/.local/share
XDG_RUNTIME_DIR=/var/folders/ws/1nchq9k97p91vbw4dtclzm_40000gn/T
XDG_STATE_HOME=/Users/vanyauhalin/.local/state
~
$ la ~ | grep "cagent"                                             15ms 20:21:57
~
$ la .config/ | grep "cagent"                                      26ms 20:21:59
# Here I open Docker Desktop app.
~
$ la ~ | grep "cagent"                                             22ms 20:22:07
drwxr-xr-x   3 vanyauhalin staff   96 Feb  7 20:22 .cagent
~
$ la .config/ | grep "cagent"                                      25ms 20:22:16
drwxr-xr-x  3 vanyauhalin staff   96 Feb  7 20:22 cagent
~

My request is, could you please support the XDG specification and use only one directory? And side question, if cagent now part of Docker Desktop, why does it create its own directory? Thank you.

https://github.com/moby/moby/issues/20693/
https://github.com/moby/moby/issues/44640/