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>

CHANGELOG.md

@@ -18,6 +18,9 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Added
 
+- Find-in-files sidebar panel (Ctrl/Cmd+Shift+F): search the workspace with
+  regex and case toggles plus include/exclude globs; results stream in grouped
+  by file and clicking a match opens the editor at that line. (T-52)
 - Workspace content-search engine with `search.grep` / `search.cancel` commands:
   a pure-Dart, isolate-parallel grep (literal or regex, case + include/exclude
   glob filters) that streams matches and honours the `ignore_files:` chain. The