agent/api: carry readout alongside streamed tokens

StreamToken::Token is now a struct variant with an optional
TokenReadout (shape [n_layers][n_concepts]) per token — parsed from
the vLLM completion response's choices[i].readout field when the
server has readout enabled.

ApiClient gains a fetch_readout_manifest() method that hits
GET /v1/readout/manifest. Returns Ok(None) on 404 (server has
readout disabled), so callers can gracefully fall back when pointed
at a non-readout-enabled endpoint.

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

src/agent/context.rs

@@ -682,7 +682,7 @@ impl ResponseParser {
             let mut full_text = String::new();
             while let Some(event) = stream.recv().await {
                 match event {
-                    super::api::StreamToken::Token(id) => {
+                    super::api::StreamToken::Token { id, readout: _ } => {
                         let text = super::tokenizer::decode(&[id]);
                         full_text.push_str(&text);
                         let mut ctx = agent.context.lock().await;