feat: convert to workspace with native plugin architecture

BREAKING: Restructure from monolithic binary to modular plugin ecosystem

Architecture changes:
- Convert to Cargo workspace with crates/ directory
- Create owlry-plugin-api crate with ABI-stable interface (abi_stable)
- Move core binary to crates/owlry/
- Extract providers to native plugin crates (13 plugins)
- Add owlry-lua crate for Lua plugin runtime

Plugin system:
- Plugins loaded from /usr/lib/owlry/plugins/*.so
- Widget providers refresh automatically (universal, not hardcoded)
- Per-plugin config via [plugins.<name>] sections in config.toml
- Backwards compatible with [providers] config format

New features:
- just install-local: build and install core + all plugins
- Plugin config: weather and pomodoro read from [plugins.*]
- HostAPI for plugins: notifications, logging

Documentation:
- Update README with new package structure
- Add docs/PLUGINS.md with all plugin documentation
- Add docs/PLUGIN_DEVELOPMENT.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -1,3 +1,13 @@
 /target
 CLAUDE.md
 media.md
+
+# AUR packages (each is its own git repo for aur.archlinux.org)
+aur/*/.git/
+aur/*/pkg/
+aur/*/src/
+aur/*/*.tar.zst
+aur/*/*.tar.gz
+aur/*/*.tar.xz
+aur/*/*.pkg.tar.*
+# Keep PKGBUILD and .SRCINFO tracked