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/server/src/shared/messages.ts

@@ -727,6 +727,13 @@ export const GitHubIssueAttachmentSchema = z.object({
   body: z.string().nullable().optional(),
 });
 
+export const TextAttachmentSchema = z.object({
+  type: z.literal("text"),
+  mimeType: z.literal("text/plain"),
+  title: z.string().nullable().optional(),
+  text: z.string(),
+});
+
 export const ReviewAttachmentContextLineSchema = z.object({
   oldLineNumber: z.number().int().positive().nullable(),
   newLineNumber: z.number().int().positive().nullable(),
@@ -758,6 +765,7 @@ export const ReviewAttachmentSchema = z.object({
 export const AgentAttachmentSchema = z.discriminatedUnion("type", [
   GitHubPrAttachmentSchema,
   GitHubIssueAttachmentSchema,
+  TextAttachmentSchema,
   ReviewAttachmentSchema,
 ]);