Add a local YAML config file for persistent settings

Reads $XDG_CONFIG_HOME/slocker-lite/config.yaml (falling back to
$HOME/.config/slocker-lite/config.yaml), organized into sections. Only
the "global" section's log-level is supported for now -- other options
are one-shot flags, not standing preferences. An explicit --log-level
on the command line always overrides the config file, the same way
SPDLOG_LEVEL already does.

Uses libyaml directly (yaml_dep was already declared in meson.build but
unused). A missing config file isn't an error; unknown sections/keys
are ignored for forward-compatibility; malformed YAML syntax is a hard
error.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gv3s5jckJKzh6JkMoi2Akz

README.md

@@ -94,6 +94,23 @@ sudo ./buildDir/slocker-lite -r myimage.tar --user git
 ./buildDir/slocker-lite -l ./images
 ```
 
+## Configuration
+
+Persistent settings can be kept in a local YAML config file at
+`$XDG_CONFIG_HOME/slocker-lite/config.yaml` (falling back to
+`$HOME/.config/slocker-lite/config.yaml` if `XDG_CONFIG_HOME` isn't set). The file is
+organized into sections; only `global` exists today:
+
+```yaml
+global:
+  log-level: debug
+```
+
+Only options that make sense as a standing preference are supported here — right now
+just `log-level` (one-shot commands like `--mount`/`--run`/`--user` don't belong in a
+config file). A missing config file is fine (nothing is overridden); an explicit
+`--log-level` on the command line always overrides the config file.
+
 ## How it works
 
 Image layers are imported into `containers-storage` (parent-chained) and the