surface agent infrastructure: hook spawn, seen set rotation, config

Surface agent fires asynchronously on UserPromptSubmit, deposits
results for the next prompt to consume.  This commit adds:

- poc-hook: spawn surface agent with PID tracking and configurable
  timeout, consume results (NEW RELEVANT MEMORIES / NO NEW), render
  and inject surfaced memories, observation trigger on conversation
  volume
- memory-search: rotate seen set on compaction (current → prev)
  instead of deleting, merge both for navigation roots
- config: surface_timeout_secs option

The .agent file and agent output routing are still pending.

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

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

@@ -110,10 +110,16 @@ fn main() {
     let is_first = !cookie_path.exists();
 
     if is_first || is_compaction {
-        // Reset seen set and returned list
+        // Rotate seen set: current → prev (for surface agent navigation roots)
         let seen_path = state_dir.join(format!("seen-{}", session_id));
+        let seen_prev_path = state_dir.join(format!("seen-prev-{}", session_id));
         let returned_path = state_dir.join(format!("returned-{}", session_id));
-        fs::remove_file(&seen_path).ok();
+        if is_compaction {
+            fs::rename(&seen_path, &seen_prev_path).ok();
+        } else {
+            fs::remove_file(&seen_path).ok();
+            fs::remove_file(&seen_prev_path).ok();
+        }
         fs::remove_file(&returned_path).ok();
     }
 
@@ -592,6 +598,35 @@ fn parse_seen_line(line: &str) -> &str {
     line.split_once('\t').map(|(_, key)| key).unwrap_or(line)
 }
 
+/// Load the most recently surfaced memory keys, sorted newest-first, capped at `limit`.
+/// Used to give the surface agent navigation roots.
+fn load_recent_seen(dir: &Path, session_id: &str, limit: usize) -> Vec<String> {
+    // Merge current and previous seen sets
+    let mut entries: Vec<(String, String)> = Vec::new();
+    for suffix in ["", "-prev"] {
+        let path = dir.join(format!("seen{}-{}", suffix, session_id));
+        if let Ok(content) = fs::read_to_string(&path) {
+            entries.extend(
+                content.lines()
+                    .filter(|s| !s.is_empty())
+                    .filter_map(|line| {
+                        let (ts, key) = line.split_once('\t')?;
+                        Some((ts.to_string(), key.to_string()))
+                    })
+            );
+        }
+    }
+
+    // Sort by timestamp descending (newest first), dedup by key
+    entries.sort_by(|a, b| b.0.cmp(&a.0));
+    let mut seen = HashSet::new();
+    entries.into_iter()
+        .filter(|(_, key)| seen.insert(key.clone()))
+        .take(limit)
+        .map(|(_, key)| key)
+        .collect()
+}
+
 fn load_seen(dir: &Path, session_id: &str) -> HashSet<String> {
     let path = dir.join(format!("seen-{}", session_id));
     if path.exists() {