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/update.go

@@ -21,6 +21,9 @@ type tickMsg struct{}
 // copiedFlashFrames is how many ticks the "✓ copied" footer note lingers after a yank.
 const copiedFlashFrames = 12
 
+// fileListMax caps how many @-picker rows are filtered/rendered at once.
+const fileListMax = 200
+
 func readServer(c *ws.Client) tea.Cmd {
 	return func() tea.Msg { return serverMsg{<-c.Incoming()} }
 }
@@ -51,7 +54,7 @@ func (m Model) animating() bool {
 	if m.reconnecting {
 		return true
 	}
-	if m.overlay == OverlayPalette { // the palette shows a blinking filter caret
+	if m.overlay == OverlayPalette || m.overlay == OverlayFiles { // blinking filter caret
 		return true
 	}
 	if m.copiedFlash != 0 && m.frame-m.copiedFlash < copiedFlashFrames {
@@ -448,6 +451,12 @@ func (m Model) handleInsertKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 			m.openPalette()
 			return m, nil
 		}
+		// `@` at a token boundary opens the workspace file picker; the selection inserts
+		// `@path`. Mid-word `@` (e.g. an email) is literal.
+		if m.inputMode == ModeRouter && string(k.Runes) == "@" && m.atTokenBoundary() {
+			m.openFiles()
+			return m, nil
+		}
 		m.appendRunes(string(k.Runes))
 		m.syncFilter()
 	}
@@ -462,6 +471,65 @@ func (m *Model) openPalette() {
 	m.paletteIndex = 0
 }
 
+// atTokenBoundary reports whether the input cursor sits at the start of a word (start of
+// line or just after a space) — where `@` should begin a file reference rather than be literal.
+func (m Model) atTokenBoundary() bool {
+	if m.inputCursor <= 0 {
+		return true
+	}
+	if m.inputCursor <= len(m.inputBuffer) {
+		return m.inputBuffer[m.inputCursor-1] == ' '
+	}
+	return false
+}
+
+// openFiles opens the `@` file picker for the selected session and requests its workspace
+// paths (cached per session). Editing mode is left in Insert so closing returns to typing.
+func (m *Model) openFiles() {
+	m.overlay = OverlayFiles
+	m.fileFilter = ""
+	m.fileIndex = 0
+	if m.filesFor != m.selectedID {
+		m.files = nil
+		m.filesLoading = true
+	}
+	m.client.Send(protocol.ListFiles(m.selectedID))
+}
+
+// insertFileRef inserts `@path ` at the input cursor (the `@` was consumed to open the picker).
+func (m *Model) insertFileRef(path string) {
+	ref := "@" + path + " "
+	c := m.inputCursor
+	if c > len(m.inputBuffer) {
+		c = len(m.inputBuffer)
+	}
+	m.inputBuffer = m.inputBuffer[:c] + ref + m.inputBuffer[c:]
+	m.inputCursor = c + len(ref)
+	m.fileFilter = ""
+}
+
+// filteredFiles narrows the workspace paths by the typed filter (case-insensitive substring),
+// capped for render.
+func (m Model) filteredFiles() []string {
+	if m.fileFilter == "" {
+		if len(m.files) > fileListMax {
+			return m.files[:fileListMax]
+		}
+		return m.files
+	}
+	f := strings.ToLower(m.fileFilter)
+	out := make([]string, 0, fileListMax)
+	for _, p := range m.files {
+		if strings.Contains(strings.ToLower(p), f) {
+			out = append(out, p)
+			if len(out) >= fileListMax {
+				break
+			}
+		}
+	}
+	return out
+}
+
 // beginIntent stashes the chosen workflow and switches to a text-entry prompt for the
 // freeform request. Submitting sends StartSession(id, intent); an empty line starts with no
 // intent (fixed-task workflows). Esc cancels.
@@ -638,6 +706,31 @@ func (m Model) handleOverlayKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 		case runeIs(k, "I"):
 			m.overlay = OverlayNone
 		}
+	case OverlayFiles:
+		files := m.filteredFiles()
+		switch {
+		case k.Type == tea.KeyUp:
+			if m.fileIndex > 0 {
+				m.fileIndex--
+			}
+		case k.Type == tea.KeyDown:
+			if m.fileIndex < len(files)-1 {
+				m.fileIndex++
+			}
+		case k.Type == tea.KeyEnter:
+			if m.fileIndex >= 0 && m.fileIndex < len(files) {
+				m.insertFileRef(files[m.fileIndex])
+			}
+			m.overlay = OverlayNone // back to typing (still in insert mode)
+		case k.Type == tea.KeyBackspace:
+			if n := len(m.fileFilter); n > 0 {
+				m.fileFilter = m.fileFilter[:n-1]
+				m.fileIndex = 0
+			}
+		case k.Type == tea.KeyRunes || k.Type == tea.KeySpace:
+			m.fileFilter += string(k.Runes)
+			m.fileIndex = 0
+		}
 	}
 	return m, nil
 }