update: session-grouping review follow-up

- PickerOutcome::Picked から未使用の session_id を除去（pod-cli が lookup_session_of で再解決）
- picker preview が singular AssistantItem も拾うように
- fs_store layout doc に migration（後方互換なし、旧 flat sessions は破棄）を明記
- TaskStore は Session-lifetime、ScopedFs/Tracker は Pod-process lifetime と用語整理
- Pod::session_id / from_manifest_spawned のコメント補強

crates/tui/src/main.rs

@@ -210,7 +210,7 @@ async fn run_resume() -> Result<(), Box<dyn std::error::Error>> {
     // viewport before the name dialog opens so each phase gets fresh
     // vertical room.
     let leaf_segment_id = match picker::run().await? {
-        PickerOutcome::Picked { segment_id, .. } => segment_id,
+        PickerOutcome::Picked { segment_id } => segment_id,
         PickerOutcome::Cancelled => return Ok(()),
     };
     run_spawn(Some(leaf_segment_id)).await

crates/tui/src/picker.rs

@@ -62,11 +62,10 @@ impl From<session_store::StoreError> for PickerError {
 }
 
 pub enum PickerOutcome {
-    /// User picked a session; resume at its leaf segment.
-    Picked {
-        session_id: SessionId,
-        segment_id: SegmentId,
-    },
+    /// User picked a session; resume at its leaf segment. The pod-cli
+    /// rehydrates `session_id` via `Store::lookup_session_of` so we only
+    /// need to surface the segment here.
+    Picked { segment_id: SegmentId },
     Cancelled,
 }
 
@@ -134,10 +133,8 @@ pub async fn run() -> Result<PickerOutcome, PickerError> {
             }
             Some(Action::Submit) => {
                 close_viewport(&mut terminal)?;
-                let row = &rows[selected];
                 return Ok(PickerOutcome::Picked {
-                    session_id: row.session_id,
-                    segment_id: row.leaf_segment_id,
+                    segment_id: rows[selected].leaf_segment_id,
                 });
             }
             Some(Action::Cancel) => {
@@ -200,6 +197,11 @@ fn last_message_preview(entries: &[LogEntry]) -> Option<String> {
                     return Some(format!("user: {}", trim_one_line(&text, 60)));
                 }
             }
+            LogEntry::AssistantItem { item, .. } => {
+                if let Some(text) = first_text_logged(item) {
+                    return Some(format!("assistant: {}", trim_one_line(&text, 60)));
+                }
+            }
             LogEntry::AssistantItems { items, .. } => {
                 if let Some(text) = items.iter().find_map(first_text_logged) {
                     return Some(format!("assistant: {}", trim_one_line(&text, 60)));