obs(session_store): log every record/lookup/remove for warm-resume diagnostics

Helps explain WHY each spawn goes warm vs cold. The existing
`spawn mode=warm|cold` log only shows the outcome at the spawn point —
to count where warmth is being lost, we need to see:
  - when a session_id is recorded (and for which key),
  - what every lookup returns (key + Some/None),
  - when remove_sessions_for_story prunes (which is currently the only
    explicit cold-induction path beyond "first ever spawn").

After this lands a grep of "session_store" in the logs gives the full
warm-resume health picture: which (story,agent,model) triples have a
recorded session, which lookups are hitting it, and which prunes are
costing us a warm respawn.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/src/agents/session_store.rs

@@ -12,6 +12,8 @@
 use std::collections::HashMap;
 use std::path::Path;
 
+use crate::slog;
+
 /// Composite key for the session store: `{story_id}:{agent_name}:{model}`.
 fn session_key(story_id: &str, agent_name: &str, model: &str) -> String {
     format!("{story_id}:{agent_name}:{model}")
@@ -55,8 +57,9 @@ pub fn record_session(
 ) {
     let key = session_key(story_id, agent_name, model);
     let mut data = read_store(project_root);
-    data.insert(key, session_id.to_string());
+    data.insert(key.clone(), session_id.to_string());
     write_store(project_root, &data);
+    slog!("[agents] session_store record key={key} session={session_id}");
 }
 
 /// Look up the last session_id for a (story_id, agent_name, model) triple.
@@ -70,7 +73,12 @@ pub fn lookup_session(
     model: &str,
 ) -> Option<String> {
     let key = session_key(story_id, agent_name, model);
-    read_store(project_root).get(&key).cloned()
+    let result = read_store(project_root).get(&key).cloned();
+    match &result {
+        Some(sid) => slog!("[agents] session_store lookup key={key} => found session={sid}"),
+        None => slog!("[agents] session_store lookup key={key} => no record"),
+    }
+    result
 }
 
 /// Remove all session entries for a story.
@@ -84,8 +92,12 @@ pub fn remove_sessions_for_story(project_root: &Path, story_id: &str) {
     let prefix = format!("{story_id}:");
     let before = data.len();
     data.retain(|k, _| !k.starts_with(&prefix));
-    if data.len() < before {
+    let removed = before - data.len();
+    if removed > 0 {
         write_store(project_root, &data);
+        slog!("[agents] session_store removed {removed} key(s) for story={story_id}");
+    } else {
+        slog!("[agents] session_store remove for story={story_id}: no entries matched");
     }
 }