feat(tui-go): @ file-reference picker

Typing `@` at a word boundary in the chat input (mid-word `@` stays literal, e.g. emails)
opens a workspace file picker: it requests file.list for the session (cached per session),
filters as you type, and enter inserts `@path ` at the cursor — back to typing. Renders as
a transparent overlay, windowed around the selection. file.list is classified non-rendering
in the render-matrix guard. Tests cover token-boundary detection, ref insertion, and filtering.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/tui-go/internal/app/server.go

@@ -326,6 +326,13 @@ func (m *Model) applyServer(msg protocol.ServerMessage) {
 		if m.ideasIndex >= len(m.ideas) {
 			m.ideasIndex = 0
 		}
+	case protocol.TypeFileList:
+		m.files = msg.Paths
+		m.filesFor = msg.SessionID
+		m.filesLoading = false
+		if m.fileIndex >= len(m.filteredFiles()) {
+			m.fileIndex = 0
+		}
 	case protocol.TypeConfigSnapshot:
 		m.configFields = msg.ConfigFields
 		m.configRestart = msg.ConfigRestartRequired
@@ -371,7 +378,7 @@ func sessionIDOf(msg protocol.ServerMessage) string {
 		protocol.TypeWorkflowList, protocol.TypeRouterResponse,
 		protocol.TypeModelChanged, protocol.TypeModelList, protocol.TypeResourceStatus,
 		protocol.TypeArtifactList, protocol.TypeConfigSnapshot, protocol.TypeSessionStats,
-		protocol.TypeIdeaList:
+		protocol.TypeIdeaList, protocol.TypeFileList:
 		return ""
 	default:
 		return msg.SessionID