fix: cap distiller startup scan and switch to formal SDK module dependency

- replace vendored third_party/homeagent-sdk with formal module dependency
- keep canonical SDK via go.mod pinned commit
- optimize distiller startup loading to stream recent raw records only
- parse timestamps correctly and cap startup load to 5000 records
- remove quadratic string building in raw record parsing
- restore fast startup while preserving memory pipeline

third_party/homeagent-sdk/sdk/knowledge.go

@@ -1,14 +0,0 @@
-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"`
-}