- Extend core/plugin-sdk types with runtime manifest metadata, runtime factory, and runtime registration exports
- Add runtime validation in plugin manifest parsing, including runtimeId slug and semver checks
- Add PluginLoader.getPluginRuntimes() and PluginRunner runtime cache/invalidation plumbing across plugin lifecycle events
- Expand plugin loader/runner test coverage for runtime discovery and cache behavior, and document runtime registration in PLUGIN_AUTHORING.md
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