fix(tui): start the session a sessionless model switch defers into

Launching kigi in a non-project directory (~/Downloads, ~, /tmp — anywhere
`is_project_dir` rejects) leaves the agent view session-less behind the
project-picker question, which only a PLAIN prompt can open. Slash
commands still execute there, so `/model <name> <effort>` stashed its
switch in `deferred_model_switch` — a stash that assumes a create is in
flight — and with none pending it dangled forever with zero feedback:
the picker rendered, the user chose model and effort, and nothing
changed. Not a 0.1.10 regression: 0.1.9 pty-reproduces identically; the
report correlated with the upgrade only because every earlier launch
happened to be from a project directory.

Apply the QueueCommand precedent ("queued slash work bypasses the
picker, so create the deferred session or it never drains") to both
deferral sites — Action::SwitchModel in the router and
set_default_model's no-session branch — via
skip_picker_and_create_session, whose in-flight guard already makes it a
no-op while a create is pending, so the racing-create case is unchanged.
SessionCreated then applies the stash through the existing
apply_deferred_model_switch path; pty-verified end-to-end from
~/Downloads (session created, model changed, effort applied).

crates/codegen/kigi-tui/src/app/dispatch/router.rs

@@ -62,6 +62,7 @@ use super::session::lifecycle::{
     clear_startup_actions, dispatch_agent_type_mismatch_answered, dispatch_exit_session,
     dispatch_new_session, dispatch_new_session_inner, dispatch_new_session_with_id,
     dispatch_new_worktree_session, dispatch_trust_folder, open_new_session_question,
+    skip_picker_and_create_session,
 };
 use super::session::load::{
     dispatch_cycle_session_source_filter, dispatch_load_session, dispatch_pick_content_session,
@@ -749,7 +750,12 @@ pub(crate) fn dispatch(action: Action, app: &mut AppView) -> Vec<Effect> {
             };
             let Some(session_id) = agent.session.session_id.clone() else {
                 agent.session.deferred_model_switch = Some((model_id, effort));
-                return vec![];
+                // No session bound: with a create in flight this is a no-op
+                // and `SessionCreated` applies the stash; with none in flight
+                // (project question pending — only a plain prompt opens it) a
+                // switch would dangle forever, so start the session like the
+                // QueueCommand arm does for queued slash work.
+                return skip_picker_and_create_session(app, id);
             };
             agent.session.model_switch_pending = true;
             vec![Effect::SwitchModel {