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>

examples/plugins/hello-world/plugin.toml

@@ -0,0 +1,22 @@
+# Hello World Plugin for Owlry
+#
+# This is a minimal example plugin demonstrating:
+# - Plugin manifest structure
+# - Static provider registration
+# - Returning items from Lua
+
+[plugin]
+id = "hello-world"
+name = "Hello World"
+version = "1.0.0"
+description = "A minimal example plugin"
+author = "Owlry Team"
+license = "GPL-3.0-or-later"
+owlry_version = ">=0.3.0"
+
+[provides]
+providers = ["greeting"]
+
+# No special permissions needed for this simple plugin
+[permissions]
+network = false