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/indexer.go

@@ -0,0 +1,17 @@
+package sdk
+
+// IndexerAPI exposes the memory context indexer.
+type IndexerAPI interface {
+	BuildContext(q string) (*IndexContext, error)
+	FormatContext(ctx *IndexContext) string
+	GetToolDefinitions() []map[string]interface{}
+	BuildToolPrompt() string
+}
+
+// IndexContext is a neutral snapshot of the injected memory context.
+type IndexContext struct {
+	Entities      []Entity `json:"entities"`
+	Relations     []Relation `json:"relations"`
+	Summary       string     `json:"summary"`
+	TokenEstimate int        `json:"token_estimate"`
+}