fix(cli): implement /session list in resume mode — ROADMAP #21 partial

/session list previously returned 'unsupported resumed slash command' in
--output-format json --resume mode. It only reads the sessions directory
so does not need a live runtime session.

Adds a Session{action:"list"} arm in run_resume_command() before the
unsupported catchall. Emits:
  {kind:session_list, sessions:[...ids], active:<current-session-id>}

159 CLI tests pass.

rust/crates/rusty-claude-cli/src/main.rs

@@ -2964,6 +2964,25 @@ fn run_resume_command(
             })
         }
         SlashCommand::Unknown(name) => Err(format_unknown_slash_command(name).into()),
+        // /session list can be served from the sessions directory without a live session.
+        SlashCommand::Session {
+            action: Some(ref act),
+            ..
+        } if act == "list" => {
+            let sessions = list_managed_sessions().unwrap_or_default();
+            let session_ids: Vec<String> = sessions.iter().map(|s| s.id.clone()).collect();
+            let active_id = session.session_id.clone();
+            let text = render_session_list(&active_id).unwrap_or_else(|e| format!("error: {e}"));
+            Ok(ResumeCommandOutcome {
+                session: session.clone(),
+                message: Some(text),
+                json: Some(serde_json::json!({
+                    "kind": "session_list",
+                    "sessions": session_ids,
+                    "active": active_id,
+                })),
+            })
+        }
         SlashCommand::Bughunter { .. }
         | SlashCommand::Commit { .. }
         | SlashCommand::Pr { .. }