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/init.lua

@@ -0,0 +1,64 @@
+-- Hello World Plugin for Owlry
+--
+-- This minimal plugin demonstrates:
+-- 1. Using owlry.log for logging
+-- 2. Registering a static provider
+-- 3. Returning items with all available fields
+
+owlry.log.info("Hello World plugin loading...")
+
+-- Register a static provider
+-- Static providers have a 'refresh' function that returns items once
+owlry.provider.register({
+    -- Required: unique provider name (used internally)
+    name = "greeting",
+
+    -- Optional: human-readable display name
+    display_name = "Hello World",
+
+    -- Optional: icon name (freedesktop icon spec)
+    default_icon = "face-smile",
+
+    -- Optional: prefix to trigger this provider (e.g., ":hello")
+    prefix = ":hello",
+
+    -- Required for static providers: function that returns items
+    refresh = function()
+        -- Get username from environment or default to "User"
+        local user = os.getenv("USER") or "User"
+
+        return {
+            {
+                id = "greeting-1",
+                name = "Hello, " .. user .. "!",
+                description = "A friendly greeting from Lua",
+                icon = "face-smile",
+                -- Command to run when selected (optional)
+                command = "notify-send 'Hello' 'Greetings from Owlry!'",
+                -- Whether to run in terminal (optional, default false)
+                terminal = false,
+                -- Tags for search/filtering (optional)
+                tags = { "greeting", "hello", "example" }
+            },
+            {
+                id = "greeting-2",
+                name = "Current time: " .. os.date("%H:%M:%S"),
+                description = "The current system time",
+                icon = "appointment-soon",
+                -- Empty command = info only, no action
+                command = "",
+                tags = { "time", "clock" }
+            },
+            {
+                id = "greeting-3",
+                name = "Open Owlry docs",
+                description = "Visit the Owlry documentation",
+                icon = "help-browser",
+                command = "xdg-open 'https://github.com/Owlibou/owlry'",
+                tags = { "help", "docs", "documentation" }
+            }
+        }
+    end
+})
+
+owlry.log.info("Hello World plugin loaded successfully!")