tui: dedicated code_exec renderer — highlighted code + structured result ...

code_exec went through the generic tool renderer: the whole script was dumped
as an escaped JSON string (unreadable), and the result was a dim wall of text
with stdout/stderr fused by a [stderr] marker. Give it a first-class card:

started: Python syntax highlighting via the shared highlight_code (follows
Theme.code_theme), long scripts folded at 60 lines, working_dir/timeout shown
as compact key/values instead of JSON bulk.

result: exit code anchored in the title (exit N), stdout and stderr split into
separate blocks with stderr in the warning colour, and a dedicated "timeout Ns"
status (detected from metadata or, for legacy sessions, the output text).
Registered before the generic tool renderers (first accepting wins).