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/llm.go

@@ -1,8 +0,0 @@
-package sdk
-
-// LLMAPI provides access to the LLM provider manager.
-type LLMAPI interface {
-	ListSources() []string
-	SetSource(name string) error
-	CurrentSource() string
-}