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>

CHANGELOG.md

@@ -128,6 +128,9 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Fixed
 
+- In Vim mode, Esc in insert/visual mode returns to normal mode instead of
+  closing the editor. The global "exit focus / close editor" Esc binding now
+  stands down while Vim is in insert or visual mode. (T-257)
 - The permission-mode badge and Ctrl/Cmd+M now visibly cycle the mode in the
   status line. The mode was changed on the session but never reflected back, so
   both looked dead. (T-250)