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/subconscious/generate.rs

@@ -36,7 +36,7 @@ where F: FnMut(&AstNode) -> bool,
     let mut tokens = Vec::new();
     while let Some(tok) = rx.recv().await {
         match tok {
-            StreamToken::Token(id) => tokens.push(id),
+            StreamToken::Token { id, .. } => tokens.push(id),
             StreamToken::Done { .. } => break,
             StreamToken::Error(e) => anyhow::bail!("generation error: {}", e),
         }