semi implemented and features implemented save checkpoint

docs/features/unchecked/plugin/plugin-discovery.md

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+# Plugin Discovery (FileSystem and Embedded)
+
+## Module
+Plugin
+
+## Status
+IMPLEMENTED
+
+## Description
+Multi-strategy plugin discovery with filesystem scanning, embedded plugins, and composite discovery that combines both approaches.
+
+## Implementation Details
+- **CompositePluginDiscovery**: `src/Plugin/StellaOps.Plugin.Host/Discovery/CompositePluginDiscovery.cs` -- combines multiple IPluginDiscovery sources; deduplicates by plugin ID (first-wins); supports DiscoverAsync (bulk) and DiscoverSingleAsync (by PluginSource); routes FileSystem/Embedded source types to appropriate discoverer
+- **FileSystemPluginDiscovery**: `src/Plugin/StellaOps.Plugin.Host/Discovery/FileSystemPluginDiscovery.cs` -- scans filesystem directories for plugin assemblies and manifests
+- **EmbeddedPluginDiscovery**: `src/Plugin/StellaOps.Plugin.Host/Discovery/EmbeddedPluginDiscovery.cs` -- discovers plugins embedded in host assemblies
+- **IPluginDiscovery**: `src/Plugin/StellaOps.Plugin.Host/Discovery/IPluginDiscovery.cs` -- interface: DiscoverAsync, DiscoverSingleAsync
+- **PluginManifest**: `src/Plugin/StellaOps.Plugin.Abstractions/Manifest/PluginManifest.cs` -- manifest model with Info, Dependencies, Capabilities
+- **Source**: Feature matrix scan
+
+## E2E Test Plan
+- [ ] Verify filesystem discovery scans configured paths and finds plugin assemblies
+- [ ] Test embedded discovery locates plugins within host assemblies
+- [ ] Verify composite discovery deduplicates plugins by ID across sources
+- [ ] Test single plugin discovery routes to correct discoverer by source type
+- [ ] Verify error in one discoverer does not block others