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/overlays.go

@@ -124,6 +124,8 @@ func (m Model) renderOverlay(base string) string {
 		return m.center(m.ideasModal())
 	case OverlaySessions:
 		return m.center(m.sessionsModal())
+	case OverlayFiles:
+		return m.center(m.filesModal())
 	}
 	return base
 }
@@ -520,6 +522,63 @@ func (m Model) paletteModal() string {
 	return m.center(modal)
 }
 
+// filesModal is the `@` file-reference picker: a filter line over the session workspace's
+// paths, windowed around the selection; enter inserts `@path` into the chat input.
+func (m Model) filesModal() string {
+	t := m.theme
+	w := m.modalWidth()
+	files := m.filteredFiles()
+
+	var b strings.Builder
+	b.WriteString(m.titleLine("@ file reference") + "\n\n")
+
+	caret := mbg(t, " ", t.P.BgPanel)
+	if m.caretVisible() {
+		caret = lipgloss.NewStyle().Foreground(t.P.Accent).Background(t.P.BgPanel).Render("▏")
+	}
+	prompt := lipgloss.NewStyle().Foreground(t.P.Accent).Background(t.P.BgPanel).Render("@ ")
+	if m.fileFilter == "" {
+		b.WriteString(prompt + caret + mbg(t, "type to filter files…", t.P.Faint) + "\n\n")
+	} else {
+		b.WriteString(prompt + mbg(t, m.fileFilter, t.P.FgStrong) + caret + "\n\n")
+	}
+
+	switch {
+	case m.filesLoading:
+		b.WriteString(mbg(t, "  loading…", t.P.Faint) + "\n")
+	case len(m.files) == 0:
+		b.WriteString(mbg(t, "  no workspace files (is a workspace bound?)", t.P.Faint) + "\n")
+	case len(files) == 0:
+		b.WriteString(mbg(t, "  no matching files", t.P.Faint) + "\n")
+	default:
+		const maxRows = 12
+		start := 0
+		if m.fileIndex >= maxRows {
+			start = m.fileIndex - maxRows + 1
+		}
+		end := start + maxRows
+		if end > len(files) {
+			end = len(files)
+		}
+		for i := start; i < end; i++ {
+			marker := mbg(t, "  ", t.P.BgPanel)
+			fg := t.P.Fg
+			if i == m.fileIndex {
+				marker = lipgloss.NewStyle().Foreground(t.P.Accent).Background(t.P.BgPanel).Render("▌ ")
+				fg = t.P.FgStrong
+			}
+			b.WriteString(marker +
+				lipgloss.NewStyle().Foreground(fg).Background(t.P.BgPanel).Render(clip(files[i], w-6)) + "\n")
+		}
+		if len(files) > maxRows {
+			b.WriteString(mbg(t, "  "+itoa(len(files))+" matches", t.P.Faint) + "\n")
+		}
+	}
+	b.WriteString("\n" + modalHints(t, [][2]string{{"↑↓", "select"}, {"enter", "insert"}, {"esc", "close"}}))
+	modal := t.Overlay.Width(w).Render(b.String())
+	return m.center(modal)
+}
+
 func (m Model) toolPaletteModal() string {
 	t := m.theme
 	w := m.modalWidth()