feat(sessions): add Sub-agents count column to sessions list

Call discover_agents_for_session for each session and show the real
count in a new Sub-agents column. JSON and XML output updated to include
the subagents field.

Closes claudbg-xpzp

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.beans/claudbg-xpzp--sessions-list-should-show-sub-agent-run-count-per.md

@@ -1,11 +1,11 @@
 ---
 # claudbg-xpzp
 title: '`sessions list` should show sub-agent run count per session'
-status: todo
+status: completed
 type: bug
 priority: normal
 created_at: 2026-03-30T04:40:53Z
-updated_at: 2026-03-30T04:43:26Z
+updated_at: 2026-03-30T05:15:27Z
 parent: claudbg-tci9
 blocked_by:
     - claudbg-33n0
@@ -40,3 +40,7 @@ The existing `discover_agents_for_session(session_file)` function currently look
 
 - `src/commands/sessions.rs` — `list()` function
 - `src/parser/discovery.rs` — `discover_agents_for_session`
+
+## Summary of Changes
+
+In sessions.rs list(): built a session_id->file_path map from discovered sessions, then for each DB row called discover_agents_for_session to get the real count. Added 'Sub-agents' column to table headers and row data. Updated JSON and XML output to include the subagents field.

src/commands/sessions.rs

@@ -161,6 +161,12 @@ pub async fn list(opts: &crate::cli::GlobalOpts) -> Result<()> {
         crate::db::sync::ensure_synced(&db, session_ref).await?;
     }
 
+    // Build a map from session_id -> file_path for agent discovery.
+    let session_file_map: std::collections::HashMap<String, std::path::PathBuf> = sessions
+        .into_iter()
+        .map(|sr| (sr.session_id, sr.file_path))
+        .collect();
+
     // Query DB for display.
     let conn = db
         .connect()
@@ -206,37 +212,50 @@ pub async fn list(opts: &crate::cli::GlobalOpts) -> Result<()> {
             project_path
         };
 
+        // Count sub-agents for this session.
+        let agent_count = if let Some(file_path) = session_file_map.get(&session_id) {
+            crate::parser::discovery::discover_agents_for_session(file_path)
+                .unwrap_or_default()
+                .len()
+        } else {
+            0
+        };
+
         rows.push(vec![
             display_id,
             last_msg_at,
             display_path,
             model,
             message_count.to_string(),
+            agent_count.to_string(),
         ]);
     }
 
     let output = match opts.output {
         crate::cli::OutputFormat::Table => {
-            crate::output::render_table(&["ID", "Date", "Project", "Model", "Messages"], &rows)?
+            crate::output::render_table(
+                &["ID", "Date", "Project", "Model", "Messages", "Sub-agents"],
+                &rows,
+            )?
         }
         crate::cli::OutputFormat::Json => {
             let objects: Vec<serde_json::Value> = rows
                 .iter()
                 .map(|r| {
                     serde_json::json!({
-                        "id":       r[0],
-                        "date":     r[1],
-                        "project":  r[2],
-                        "model":    r[3],
-                        "messages": r[4],
-                        "subagents": 0,
+                        "id":        r[0],
+                        "date":      r[1],
+                        "project":   r[2],
+                        "model":     r[3],
+                        "messages":  r[4],
+                        "subagents": r[5],
                     })
                 })
                 .collect();
             crate::output::render_json(&objects)?
         }
         crate::cli::OutputFormat::Xml => crate::output::render_xml_rows(
-            &["session_id", "date", "project", "model", "messages"],
+            &["session_id", "date", "project", "model", "messages", "subagents"],
             &rows,
         )?,
     };