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/model.go

@@ -57,6 +57,7 @@ const (
 	OverlayStats
 	OverlayIdeas
 	OverlaySessions
+	OverlayFiles
 )
 
 // RouterEntry is one line in a session's conversation transcript.
@@ -306,6 +307,13 @@ type Model struct {
 	paletteFilter string
 	paletteIndex  int
 
+	// @ file picker (OverlayFiles) — workspace paths from the file.list reply
+	files        []string // all workspace-relative paths for filesFor
+	filesFor     string   // sessionId the file list belongs to
+	filesLoading bool
+	fileFilter   string // the query typed after @ (narrows the list)
+	fileIndex    int
+
 	// animation
 	frame   int  // tick counter; drives spinner + caret blink
 	ticking bool // true while a tick loop is scheduled. The loop is gated on animating()