Introduce the WASM plugin system foundation:
- Add extism 1.9 as an optional dependency behind `plugins-wasm` feature
- Create `src/plugins/` module with manifest types, error types, and stub host
- Add `Plugin` CLI subcommands (list, install, remove, info) behind cfg gate
- Add `PluginsConfig` to the config schema with sensible defaults
All plugin code is behind `#[cfg(feature = "plugins-wasm")]` so the default
build is unaffected.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
