Two improvements to the Code section:
- Fix: leaving the Code section and returning no longer drops the open
session's output. The selected session id is persisted to localStorage
(mirroring the terminal-height pattern) and restored on remount, so the
right-hand chat pane re-binds instead of falling back to the empty state.
- Feature: choose the coding agent's model and reasoning effort per session.
Choices are discovered live from the engine (the same list `/model` shows)
via a new `codeMode:listModelOptions` IPC, cached per agent — never
hardcoded, so they track whatever the provider currently offers. Claude
exposes model + effort as separate axes (with explicit Opus/Sonnet/Haiku
alias rows surfaced for clarity); Codex folds effort into the model id and
reports no separate effort. Selections persist on the CodeSession and are
re-applied to the ACP session each turn (best-effort), editable from both
the new-session dialog and the session header.
