feat: XDG Base Directory Specification support

[Dave93]

Problem

Currently Velos stores all data in ~/.velos/, which clutters the home directory. This goes against the XDG Base Directory Specification.

Proposed Solution

Follow XDG conventions with fallback to current behavior:

Data type	Current	XDG-compliant
Config	~/.velos/config.toml	$XDG_CONFIG_HOME/velos/config.toml (default: ~/.config/velos/)
Logs	~/.velos/logs/	$XDG_STATE_HOME/velos/logs/ (default: ~/.local/state/velos/)
Crash data	~/.velos/crashes/	$XDG_DATA_HOME/velos/crashes/ (default: ~/.local/share/velos/)
Socket	~/.velos/velos.sock	$XDG_RUNTIME_DIR/velos/velos.sock

Migration

• Auto-migrate existing ~/.velos/ on first run if XDG dirs don't exist yet
• Print a one-time notice about the migration

References

• Raised by community feedback on Reddit
• XDG spec: https://specifications.freedesktop.org/basedir-spec/latest/
• Similar implementations: cargo, rustup, most modern CLI tools