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/pod/src/pod.rs

@@ -570,7 +570,9 @@ impl<C: LlmClient, St: Store> Pod<C, St> {
     }
 
     /// The Session this Pod belongs to. Stable across compaction and
-    /// in-Session fork — only `fork` (a brand-new Session) changes it.
+    /// auto-fork (both stay within the same Session); there is no
+    /// Pod-level operation today that moves a running Pod to a different
+    /// Session.
     pub fn session_id(&self) -> SessionId {
         self.segment_state.session_id()
     }
@@ -2864,6 +2866,8 @@ impl<St: Store> Pod<Box<dyn LlmClient>, St> {
         let mut common = prepare_pod_common(&manifest, &loader, /* parse_template */ true)?;
         let skill_shadows = std::mem::take(&mut common.skill_shadows);
 
+        // A spawned child starts its own conversation, so it mints a
+        // fresh Session rather than joining the spawner's.
         let session_id = session_store::new_session_id();
         let segment_id = session_store::new_segment_id();
         let scope_allocation = pod_registry::adopt_allocation(