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(

crates/session-store/src/fs_store.rs

@@ -6,6 +6,14 @@
 //!
 //! The per-Session directory makes `list_segments(session_id)` an O(dir)
 //! scan and gives the fork tree a visible grouping in the filesystem.
+//!
+//! Migration: this layout is incompatible with the pre-`session-grouping`
+//! flat `{root}/{segment_id}.jsonl` form. Project policy is no
+//! backward compatibility — discard `~/.insomnia/sessions/` (or whatever
+//! `root` resolved to) before running the new code. `list_sessions`
+//! ignores top-level files outside session directories, so leftover
+//! flat files do not corrupt new sessions, but they are no longer
+//! enumerable by the picker.
 
 use crate::event_trace::TraceEntry;
 use crate::segment_log::LogEntry;

crates/tools/src/lib.rs

@@ -4,12 +4,12 @@
 //! `llm-worker` `Tool` infrastructure. Filesystem access is mediated by
 //! two orthogonal concerns:
 //!
-//! - [`ScopedFs`] — pod-lifetime, expresses the write-block boundary for
-//!   the current scope. Derived from the manifest and shareable across
-//!   sessions.
-//! - [`Tracker`] — session-lifetime, enforces the "read before edit"
+//! - [`ScopedFs`] — Pod-process lifetime, expresses the write-block
+//!   boundary for the current scope. Derived from the manifest; not
+//!   persisted across Pod restart.
+//! - [`Tracker`] — Pod-process lifetime, enforces the "read before edit"
 //!   policy via content hashes and tracks the recency of touched files.
-//!   Recreated fresh per session.
+//!   Recreated fresh on each Pod start (including resume).
 //!
 //! The Pod layer owns both instances and passes them to
 //! [`builtin_tools`] when registering tools on a `Worker`.
@@ -42,11 +42,11 @@ pub use tracker::Tracker;
 pub use write::write_tool;
 
 /// Register all builtin tools, wiring them to a shared `ScopedFs`
-/// (pod-lifetime) and `Tracker` (session-lifetime).
+/// (Pod-process lifetime) and `Tracker` (Pod-process lifetime).
 ///
 /// All returned factories share the same tracker instance so that
 /// `Read` / `Write` / `Edit` see a consistent history across tool
-/// invocations within a single session.
+/// invocations within a single Pod run.
 ///
 /// `bash_output_dir` is where the Bash tool spills long outputs. The
 /// caller is responsible for adding that path to the readable scope

crates/tools/src/task.rs

@@ -1,8 +1,9 @@
-//! Pod-lifetime TaskStore and builtin task tools.
+//! Session-lifetime TaskStore and builtin task tools.
 //!
-//! The store is Pod/session-lifetime state shared by the four Task* tools. It
-//! is reconstructed on resume by replaying TaskCreate / TaskUpdate tool-call
-//! arguments from persisted history.
+//! The store survives compaction and Pod restart — it is reconstructed
+//! on resume by replaying TaskCreate / TaskUpdate tool-call arguments
+//! from persisted history, so its effective lifetime is the
+//! [`session_store::SessionId`] (the conversation), not the Pod process.
 
 use std::sync::{Arc, Mutex};
 

crates/tools/src/tracker.rs

@@ -18,11 +18,13 @@
 //!
 //! # Lifetime
 //!
-//! A `Tracker` is **session-scoped**: the Pod layer creates a fresh
-//! instance at the start of each agent session and discards it when the
-//! session ends. The `ScopedFs` write boundary, by contrast, is
-//! pod-lifetime (derived from the manifest). The two are orthogonal and
-//! the Pod wires them together when registering builtin tools.
+//! A `Tracker` is **Pod-process scoped**: the Pod layer creates a fresh
+//! instance at the start of each Pod run (including resume) and discards
+//! it when the process exits — it is not persisted, so a resumed
+//! conversation starts with an empty read/edit history. The `ScopedFs`
+//! write boundary is likewise Pod-process scoped (derived from the
+//! manifest). The two are orthogonal and the Pod wires them together
+//! when registering builtin tools.
 //!
 //! ```no_run
 //! # use std::path::PathBuf;

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)));