feat: multi-platform .hmap bundle + platform auto-selection

- PluginManifest: add platforms field declaring supported OS
- validatePackage: accept bundle .hmap with platform-aware binary check
- extractPackage: extract only current OS binary, skip others (macOS renames .dylib → .so)
- installFromPath/installFromURL: path install keeps source, URL install auto-cleanup
- cabi/loader.go + dynamic.go: add linux/darwin build constraints for Windows cross-compile
- dynamic_loader_unix/windows: split SO loading with proper build tags
- package/build.sh: enable windows/amd64 for homed, add CXX export for arm64
- tryLoadSO: fallback to plugin.dylib on macOS
- docs: update PLUGIN_DEV.md for bundle format and install methods

internal/plugin/manifest.go

@@ -19,7 +19,8 @@ type PluginManifest struct {
 	License     string   `json:"license,omitempty"`
 	Homepage    string   `json:"homepage,omitempty"`
 	Repository  string   `json:"repository,omitempty"`
-	Entry       string   `json:"entry"` // "plugin.so" | "main.lua" | "SKILL.md"
+	Entry       string   `json:"entry"` // "plugin.so" | "plugin.dll" | "main.lua" | "SKILL.md"
+	Platforms   []string `json:"platforms,omitempty"` // 声明的支持平台: ["linux","darwin","windows"]
 	MinVersion  string   `json:"min_version,omitempty"`
 	Tags        []string `json:"tags,omitempty"`
 	Deprecated  bool     `json:"deprecated,omitempty"`