fix(acp/pool): preserve session ID on session/load timeout

When session/load times out transiently, return an error to the user
instead of falling through to session/new with no history context.
The original session ID is already in state.persisted (never modified on
this code path), so the next message automatically retries session/load.

Only actual timeouts trigger this path; permanent rejections (e.g.
session/load rejected) still fall through to session/new as before.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: chenhan1218

src/acp/pool.rs

@@ -243,6 +243,7 @@ impl SessionPool {
         new_conn.initialize().await?;
 
         let mut resumed = false;
+        let mut load_failed = false;
         if let Some(ref sid) = saved_session_id {
             if new_conn.supports_load_session {
                 match new_conn.session_load(sid, &effective_workdir).await {
@@ -251,19 +252,32 @@ impl SessionPool {
                         resumed = true;
                     }
                     Err(e) => {
-                        warn!(thread_id, session_id = %sid, error = %e, "session/load failed, creating new session");
+                        let is_timeout = e.to_string().contains("timeout waiting for");
+                        if is_timeout {
+                            warn!(thread_id, session_id = %sid, error = %e,
+                                "session/load timed out, preserving session ID for retry");
+                            load_failed = true;
+                        } else {
+                            warn!(thread_id, session_id = %sid, error = %e,
+                                "session/load failed, creating new session");
+                        }
                     }
                 }
             }
         }
 
+        if load_failed {
+            // session/load timed out transiently. The original session ID is already
+            // in state.persisted (we haven't touched it), so the next message will
+            // retry session/load automatically. Return an error so the current message
+            // is not processed against a context-free session.
+            return Err(anyhow!("session load timeout: could not restore previous session"));
+        }
+
         if !resumed {
             new_conn.session_new(&effective_workdir).await?;
-            // Surface the reset banner both for restored sessions and for stale
-            // live entries that died before we could recover a resumable
-            // session id. In both cases the caller is continuing after an
-            // unexpected session loss.
             if had_existing || saved_session_id.is_some() {
+                // Genuine session loss (agent died, session file gone, etc.).
                 new_conn.session_reset = true;
             }
         }

src/error_display.rs

@@ -10,6 +10,9 @@ pub fn format_user_error(message: &str) -> String {
     let msg_lower = message.to_lowercase();
 
     // Startup / connection errors (code == 0 from anyhow)
+    if msg_lower.contains("session load timeout") {
+        return "**Session Load Timeout**\nCould not restore your previous session. Send any message to retry, or use /reset to start fresh.".to_string();
+    }
     if msg_lower.contains("timeout waiting for") {
         // Use msg_lower for extraction to stay case-insistent with the match above.
         // msg_lower and message are the same length, so byte offsets are valid.
@@ -97,6 +100,13 @@ mod tests {
 
     // ─── format_user_error tests ─────────────────────────────────────────────
 
+    #[test]
+    fn format_user_error_session_load_timeout() {
+        let result = format_user_error("session load timeout: could not restore previous session");
+        assert!(result.contains("Session Load Timeout"));
+        assert!(result.contains("/reset"));
+    }
+
     #[test]
     fn format_user_error_timeout() {
         let result = format_user_error("timeout waiting for session/new response");