fix: mark surfaced keys as returned so --seen classifies them correctly

The surface agent result consumer in poc-hook was writing to the seen
file but not the returned file, so surfaced keys showed up as
"context-loaded" in memory-search --seen.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-memory/src/bin/memory-search.rs

@@ -631,12 +631,13 @@ fn load_recent_seen(dir: &Path, session_id: &str, limit: usize) -> Vec<String> {
 fn load_seen(dir: &Path, session_id: &str) -> HashSet<String> {
     let path = dir.join(format!("seen-{}", session_id));
     if path.exists() {
-        fs::read_to_string(path)
+        let set: HashSet<String> = fs::read_to_string(&path)
             .unwrap_or_default()
             .lines()
             .filter(|s| !s.is_empty())
             .map(|s| parse_seen_line(s).to_string())
-            .collect()
+            .collect();
+        set
     } else {
         HashSet::new()
     }

poc-memory/src/bin/poc-hook.rs

@@ -247,6 +247,13 @@ fn surface_agent_cycle(hook: &Value) {
                                             let ts = chrono::Local::now().format("%Y-%m-%dT%H:%M:%S");
                                             let _ = writeln!(f, "{}\t{}", ts, key);
                                         }
+                                        let returned_path = state_dir.join(format!("returned-{}", session_id));
+                                        if let Ok(mut f) = fs::OpenOptions::new()
+                                            .create(true).append(true).open(&returned_path)
+                                        {
+                                            use std::io::Write;
+                                            let _ = writeln!(f, "{}", key);
+                                        }
                                     }
                                 }
                             }