search: exclude seen set when running in agent session

Make Session::from_env() and Session::seen() the public API for
accessing session state. Internal callers converted to use session
methods. Search automatically filters already-surfaced nodes when
POC_SESSION_ID is set.

poc-memory/src/memory_search.rs

@@ -43,6 +43,24 @@ impl Session {
     pub fn path(&self, prefix: &str) -> PathBuf {
         self.state_dir.join(format!("{}-{}", prefix, self.session_id))
     }
+
+    /// Load from POC_SESSION_ID environment variable
+    pub fn from_env() -> Option<Self> {
+        let session_id = std::env::var("POC_SESSION_ID").ok()?;
+        if session_id.is_empty() { return None; }
+        let state_dir = PathBuf::from("/tmp/claude-memory-search");
+        Some(Session {
+            session_id,
+            transcript_path: String::new(),
+            hook_event: String::new(),
+            state_dir,
+        })
+    }
+
+    /// Get the seen set for this session
+    pub fn seen(&self) -> HashSet<String> {
+        load_seen(&self.state_dir, &self.session_id)
+    }
 }
 
 /// Run the hook logic on parsed JSON input. Returns output to inject.
@@ -202,7 +220,7 @@ fn surface_agent_cycle(session: &Session, out: &mut String, log_f: &mut File) {
                 let _ = writeln!(log_f, "keys {:?}", keys);
 
                 let Ok(store) = crate::store::Store::load() else { return; };
-                let mut seen = load_seen(&session.state_dir, &session.session_id);
+                let mut seen = session.seen();
                 let seen_path = session.path("seen");
                 for key in &keys {
                     if !seen.insert(key.clone()) {
@@ -304,7 +322,7 @@ fn hook(session: &Session) -> String {
             if output.status.success() {
                 let ctx = String::from_utf8_lossy(&output.stdout).to_string();
                 if !ctx.trim().is_empty() {
-                    let mut ctx_seen = load_seen(&session.state_dir, &session.session_id);
+                    let mut ctx_seen = session.seen();
                     for line in ctx.lines() {
                         if line.starts_with("--- ") && line.ends_with(" ---") {
                             let inner = &line[4..line.len() - 4];