Move registry cache to XDG cache directory

[polymorcodeus]

Summary

Adds XDG cache directory support for the machine-local registry and moves the default cache path from ~/.config/.todocache to $XDG_CACHE_HOME/todo/registry.json (falling back to ~/.cache/todo/registry.json). This aligns the registry with XDG base-directory conventions, since the file is rebuildable cache data rather than user configuration.

What's new

• XDG cache directory support - internal/registry.DefaultPath now honors $XDG_CACHE_HOME before falling back to ~/.cache.
• Cleaner cache layout - the registry file is now ~/.cache/todo/registry.json instead of a hidden dotfile under ~/.config.
• Updated docs - README.md and CONTRIBUTING.md now describe the new location.

Behavior changes / guardrails

• The old ~/.config/.todocache file is no longer read or written. Existing entries are orphaned, but the registry is rebuildable by running todo doctor --fix.
• No automatic migration is provided because the cache can be reconstructed from disk.

Testing

• Updated registry and CLI tests to use registry.json filenames in isolated temp dirs.
• make check runs fmt, vet, lint, and tests; all green.