sdk: embed non-toolchain SDK in third_party, add NoMemory/Cleaner support

- Embed sdk/, example/, meta/, go.mod from homeagent-sdk (no .git)
- Core .gitignore excludes SDK toolchain: bin/, tools/, package/
- RegisterInputChannel + ChannelDef(NoMemory, Cleaner) in SDK
- IOManager input channel registry with GetInputChannelDef
- eventloop: apply channel Cleaner/NoMemory to interrupt text
- context engine: channelDefLookup applied in textForVector
- document store: ChannelCleaner param for archive functions
- All callers/adapters updated with ChannelDef{} default

third_party/homeagent-sdk/sdk/llm.go

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