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-rune/init.rn

@@ -0,0 +1,10 @@
+// Hello World Plugin for Owlry
+//
+// This minimal plugin demonstrates:
+// 1. Using owlry::log_* for logging
+// 2. Basic Rune syntax
+
+pub fn main() {
+    owlry::log_info("Hello Rune plugin loading...");
+    owlry::log_info("Hello Rune plugin loaded successfully!");
+}

examples/plugins/hello-rune/plugin.toml

@@ -0,0 +1,16 @@
+# Hello World Plugin for Owlry (Rune version)
+#
+# This example demonstrates a minimal Rune plugin.
+
+[plugin]
+id = "hello-rune"
+name = "Hello Rune"
+version = "1.0.0"
+description = "A simple greeting plugin written in Rune"
+author = "Owlry Team"
+license = "GPL-3.0-or-later"
+owlry_version = ">=0.3.0"
+entry = "init.rn"
+
+[provides]
+providers = ["greeting"]