fix(tui): repaint the header context window on a /model switch

After `/model`, the header kept the previous model's window ("15K /
1.0M" on a 262K model) until the next turn or a restart. The pager
caches the window inside its context snapshot (streaming `totalTokens`
updates refresh it from the catalog) and nothing refreshed the cache
when the session's model changed. Comparing cached numbers against
catalog windows cannot tell a catalog-derived total from a shell
effective limit (KIGI_DEBUG_CONTEXT_WINDOW, header upgrades), so the
shell, which installs the effective window, now reports it:

- kigi-shell: `handle_set_session_model` returns the installed window;
  `model_switch::apply` stamps it as `totalContextTokens` in the
  `session/set_model` response meta and carries it on the
  `ModelChanged` broadcast (`context_window`, serde-default: old pagers
  ignore it, old leaders omit it). The response meta previously
  serialized the actor's inner `Result` and reported success on an
  actor failure; that error now propagates.
- kigi-tui: `Effect::SwitchModel` reads the meta; `SwitchModelComplete`
  carries `Ok(Option<u64>)`; the completion handler and the follower
  `ModelChanged` arm call `AgentView::apply_context_window`, which
  re-derives the snapshot (pct, free tokens) and reports whether it
  changed so a follower redraws on a window-only change.
- Tests: the bare `/model <name>` dispatch path asserts the snapshot and
  the drawn header; follower with/without the field; same-model new
  window redraw; actor returns the window; serde round trip.

Claude-Session: https://claude.ai/code/session_01RUqV8HVGXcAETyYbZ2tPQ4

crates/codegen/kigi-shell/src/session/commands.rs

@@ -173,7 +173,9 @@ pub enum SessionCommand {
         /// `compaction.threshold_percent` (which is `Cell<u8>` so it can
         /// update without `&mut self`).
         auto_compact_threshold_percent: u8,
-        responds_to: oneshot::Sender<Result<acp::ModelId, acp::Error>>,
+        /// Ok: the applied model id and the session's effective context
+        /// window, which clients repaint from without waiting for a turn.
+        responds_to: oneshot::Sender<Result<(acp::ModelId, std::num::NonZeroU64), acp::Error>>,
     },
     /// Zero-turn harness rebuild: build a brand-new `Agent` from the
     /// session's `AgentRebuildSpec` and the new `AgentDefinition`,