* fix(coding-agent): tree selector focuses nearest visible ancestor
When the selected entry is not visible (filtered out by mode change or a
metadata entry like model_change), walk up the parent chain to find the
nearest visible ancestor instead of jumping to the last item.
Fixes selection behavior for:
- Initial selection when currentLeafId is a metadata entry
- Filter switching, e.g. Ctrl+U for user-only mode
* fix(coding-agent): tree selector preserves selection through empty filters
When switching to a filter with no results, e.g. labeled-only with no
labels and back, the cursor would reset to the first message instead of
the original selection.
Track lastSelectedId as a class member and only update it when
filteredNodes is non-empty, preserving the selection across empty filter
results.
* test(coding-agent): add tree selector filter and selection tests
- Test metadata entry handling (model_change, thinking_level_change)
- Test filter switching with parent traversal (default ↔ user-only)
- Test empty filter preservation (labeled-only with no labels)
