add cmd+k palette with chat mode that captures editor cursor context

Cmd+K (Ctrl+K on Win/Linux) now opens a unified palette with two
modes: Chat (default) and Search (existing behavior). Tab cycles
between them. In Chat mode, if the user triggered the shortcut from
the markdown editor, the palette auto-attaches a removable chip
showing the note path and precise cursor line. Enter sends the
prompt to the right-sidebar copilot — opening the sidebar if closed
and starting a fresh chat tab — with the chip carried as a
FileMention whose lineNumber is forwarded to the agent as
"... at <path> (line N)" so the agent can use workspace-readFile
with offset to fetch the right slice on demand.

Line numbers are computed against the same getMarkdownWithBlankLines
serializer used to write notes to disk, so the reference is
byte-identical to what the agent reads back.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/x/packages/shared/src/message.ts

@@ -41,6 +41,7 @@ export const UserAttachmentPart = z.object({
     filename: z.string(),                // display name ("photo.png")
     mimeType: z.string(),                // MIME type ("image/png", "text/plain")
     size: z.number().optional(),         // bytes
+    lineNumber: z.number().int().min(1).optional(),  // 1-indexed line in source file (for editor-context references)
 });
 
 // Any single part of a user message (text or attachment)