fix Esc closing the editor instead of returning to Vim normal mode

panel.focusMode.exit is bound to Escape in the contributions layer, which
outranks the active preset. In Vim insert/visual mode that shadowed the
vim preset's `escape → vim.mode.normal` binding, so Esc closed the editor
instead of returning to normal mode.

Add an optional when-clause to a CommandContribution's defaultBinding
(plumbed through to KeymapService.registerCommandBinding, which already
accepts one) and guard focusMode.exit's escape with
`!vim.insert && !vim.visual` — symmetric with vim.yaml's vim.mode.normal.
Esc still exits focus / closes the editor in normal and non-Vim modes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/default_layout/src/extension.dart

@@ -79,6 +79,10 @@ class DefaultLayoutExtension extends ClideExtension {
           command: 'panel.focusMode.exit',
           title: 'Exit Focus Mode',
           defaultBinding: 'escape',
+          // Stand down in Vim insert/visual mode so Esc returns to normal
+          // mode instead of closing the editor (T-257). Symmetric with
+          // vim.yaml's `vim.mode.normal` (escape when vim.insert||vim.visual).
+          bindingWhen: '!vim.insert && !vim.visual',
           run: _exitFocusMode,
         ),
         // Editor split (D-049, D-054)