tui: load session message history into the chat panel on resume ...

Resuming or switching a session showed a blank chat — the past conversation
was never replayed. GET /sessions/{id} already returns the messages; the TUI
just ignored them.

- ChatModel.load_history maps persisted Message dicts back to the same
  ChatItems the live stream produces (user/assistant text, thinking, tool
  started/result, plan), skipping is_display=False (context-only user,
  summaries, compression events) so only what the user would have seen
  live is rebuilt.
- ChatPanel.load_history wraps it (clears the render cache + refresh).
- attach_session loads the messages from get_session, so resume/switch
  replays the history before the cwd/Connected banners.

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

tui: bound chat history render cost (cache + visible-item cap) ...

The chat panel rebuilt every renderable on each WS event, including
per-token stream_delta — so markdown was re-parsed for the entire history
on every token and the TUI lagged as conversations grew.

- Cache renderables keyed by id(item)+signature so only the changed item
  (the streaming assistant bubble) is rebuilt per token; the rest are
  reused. Prune orphaned entries so cache stays bounded by live items.
- Add max_visible_items TUI setting (default 200): render only the last
  N items with a "… N earlier messages not shown" hint when truncated.
  Items stay in the in-memory model (non-destructive) but are not laid
  out, so per-refresh cost is constant regardless of history length.
- Rename the cache attr to _chat_render_cache to avoid collision with
  Textual Widget._render_cache, which it resets on mount.

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