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/skill.go

@@ -0,0 +1,22 @@
+package sdk
+
+// SkillAPI provides access to the skill manager.
+type SkillAPI interface {
+	List() []*Skill
+	Get(name string) *Skill
+	Install(name, content string) error
+	Uninstall(name string) error
+	Toggle(name string, enabled bool) error
+}
+
+// Skill is a neutral description of an installed skill.
+type Skill struct {
+	Name        string `json:"name"`
+	Description string `json:"description"`
+	Version     string `json:"version"`
+	Author      string `json:"author,omitempty"`
+	Entry       string `json:"entry,omitempty"`
+	Source      string `json:"source,omitempty"`
+	Enabled     bool   `json:"enabled"`
+	RawContent  string `json:"-"`
+}