wire up /score command and debug screen for memory importance

/score snapshots the context and client, releases the agent lock,
runs scoring in background. Only one score task at a time
(scoring_in_flight flag). Results stored on Agent and shown on
the F10 context debug screen with importance scores per memory.

ApiClient derives Clone. ContextState derives Clone.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/api/mod.rs

@@ -57,6 +57,7 @@ pub enum StreamEvent {
     Error(String),
 }
 
+#[derive(Clone)]
 pub struct ApiClient {
     client: Client,
     api_key: String,