add find-in-files sidebar panel + Ctrl/Cmd+Shift+F

The find-in-files UI on top of the search.grep engine. A
FindInFilesController drives search.grep, accumulates streamed
search.match events (scoped to the active searchId, stale ids
ignored) grouped by file, and opens a match in the editor at its line.
The SearchPanelView contributes a sidebar tab: a debounced query box,
regex + case toggles, include/exclude glob fields, and a grouped
results list with the matched span highlighted.

findInFiles.open (Ctrl/Cmd+Shift+F) reveals and activates the search
tab.

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

assets/keymaps/default.yaml

@@ -76,6 +76,10 @@ bindings:
     keys: escape
     when: quickOpen.open
 
+  # -- Find in files ----------------------------------------------------
+  - intent: findInFiles.open
+    keys: [ctrl+shift+f, meta+shift+f]
+
   # -- Text scale -------------------------------------------------------
   # On most layouts `+` is `shift+equal`; we bind both so users who
   # think of it as Ctrl+Plus and users who hit Ctrl+= both work.