refactor: migrate built-in plugins to SDK-only interface

- Six-phase plan complete: webui/cli/healthcheck/pluginmgr/clawhubadapter
  now interact with the kernel exclusively via internal/sdk interfaces;
  all Configure() calls and package-level global injection removed
- buildSDK in internal/plugin/registry.go is the single assembly point
- Add internal/sdk/events.go exporting event types/constants
- Fix ProviderManager cooldown sharing: LuaAdaptedProvider.Name() now
  returns the source name instead of lua_<adapter>, so multiple sources
  sharing an adapter (single script load via shared VM AdapterCache) no
  longer share failure-cooldown state
- Verified: build/vet/tests green, deployed to homeagent.service with
  full plugin capability testing via local OpenAI-compatible mock

internal/sdk/adapter.go

@@ -0,0 +1,16 @@
+package sdk
+
+// AdapterAPI provides access to the Lua LLM protocol adapters.
+type AdapterAPI interface {
+	List() []APIAdapter
+	// Load writes the adapter source file and hot-reloads it.
+	Load(name, code string) error
+	Remove(name string) error
+	AdapterDir() string
+}
+
+// APIAdapter describes a loaded Lua protocol adapter.
+type APIAdapter struct {
+	Name    string `json:"name"`
+	Version string `json:"version"`
+}