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/view.go

@@ -227,6 +227,9 @@ func (m Model) renderFooter() string {
 		if m.inputMode == ModeRouter && m.inputBuffer == "" {
 			hints = append(hints, hint("/", "cmds"))
 		}
+		if m.inputMode == ModeRouter {
+			hints = append(hints, hint("@", "files"))
+		}
 	case m.displayState() == StateApproval:
 		// Approval actions live in the band itself; the footer offers navigation only.
 		hints = []string{hint("l", "back"), hint("e", "events"), hint("q", "quit")}