fix(agent): match on Result return type from maybe_hydrate_thread

The maybe_hydrate_thread signature changed from Option<String> to
Result<Option<Uuid>, String> but the caller still pattern-matched
with Some(), causing a type mismatch clippy/compile error. Switch
to Err() to match the new error-variant semantics.

[skip-regression-check]

https://claude.ai/code/session_013ZCQWoFHv2hASgHEGHzptg

src/agent/agent_loop.rs

@@ -1010,7 +1010,7 @@ impl Agent {
                 thread_id = %external_thread_id,
                 "Hydrating thread from DB"
             );
-            if let Some(rejection) = self.maybe_hydrate_thread(message, external_thread_id).await {
+            if let Err(rejection) = self.maybe_hydrate_thread(message, external_thread_id).await {
                 return Ok(Some(format!("Error: {}", rejection)));
             }
         }