Add browser pane element picker (#670)

* feat: add browser pane element picker

Port the browser pane from PR #198 and route picked elements through workspace attachments so the context follows every agent composer in a workspace.

Co-authored-by: Jason Kneen <jason.kneen@bouncingfish.com>

* docs: document electron platform overlays

* fix: harden browser pane navigation

* fix: polish browser tab interactions

* fix: route browser pane focus shortcuts

---------

Co-authored-by: Jason Kneen <jason.kneen@bouncingfish.com>

packages/app/src/panels/pane-context.test.ts

@@ -12,6 +12,7 @@ describe("createPaneFocusContextValue", () => {
       isWorkspaceFocused: true,
       isPaneFocused: true,
       isInteractive: true,
+      focusPane: expect.any(Function),
     });
     expect(
       createPaneFocusContextValue({
@@ -22,6 +23,7 @@ describe("createPaneFocusContextValue", () => {
       isWorkspaceFocused: false,
       isPaneFocused: true,
       isInteractive: false,
+      focusPane: expect.any(Function),
     });
   });
 });