feat: converge dynamic plugins onto canonical homeagent-sdk

- Separate built-in plugin interface from external plugin interface
- Route dynamic plugin loading through homeagent-sdk/sdk using reflection
- Turn internal/sdk into an enhanced wrapper over canonical SDK types
- Vendor SDK repo snapshot under third_party/homeagent-sdk for stable builds
- Keep internal constructors/adapters for memory, knowledge, llm, settings
- Align dynamic QQ loading with canonical SDK chain

third_party/homeagent-sdk/sdk/knowledge.go

@@ -0,0 +1,14 @@
+package sdk
+
+// KnowledgeAPI provides access to the knowledge store.
+type KnowledgeAPI interface {
+	Search(query string, topK int) ([]*Knowledge, error)
+	Add(name, content string) error
+	List() ([]string, error)
+}
+
+// Knowledge represents a knowledge entry.
+type Knowledge struct {
+	Name    string `json:"name"`
+	Content string `json:"content"`
+}