feat(tui-go): make the ? overlay list every keybind

Audited handleNormalKey / handleApprovalKey / the OverlayDiff handler against the
help cheat-sheet and found gaps: enter, ↑↓/jk, /, s, c, a, w, q and ? itself were
bound but undocumented. Reorganise helpBody into navigate / session / overlays /
approval band / diff-viewer groups covering all of them. help_complete_test.go
asserts a row exists for every binding so the help can't silently drift from the
handlers. The longer sheet scrolls fine via the modal-scroll support.

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

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

@@ -818,20 +818,31 @@ func (m Model) helpBody() []string {
 		}
 		out = append(out, "")
 	}
-	section("session", []kb{
-		{"i", "compose a message"},
+	section("navigate", []kb{
+		{"↑↓ / j k", "move the session / list selection"},
+		{"enter", "open the selected session"},
+		{"/", "filter the session list"},
 		{"^↑ / ^↓", "jump to your previous / next message"},
-		{"y", "copy the selected message (OSC52)"},
 		{"PgUp / PgDn", "scroll output (^u / ^d half-page)"},
 		{"esc", "drop selection / scroll → follow newest"},
-		{"^x", "open the latest diff"},
 		{"l", "back to the session list"},
 	})
+	section("session", []kb{
+		{"i", "compose a message"},
+		{"s", "toggle chat / steering mode"},
+		{"w", "show workflows (idle screen)"},
+		{"y", "copy the selected message (OSC52)"},
+		{"^x", "open the latest diff / preview"},
+		{"c", "cancel the running session"},
+		{"a", "re-open a dismissed approval gate"},
+		{"q", "quit"},
+	})
 	section("overlays", []kb{
-		{"p", "command palette (shows all shortcuts)"},
+		{"p", "command palette (all shortcuts)"},
+		{"?", "this keybindings help"},
 		{"e", "event inspector (/ to filter)"},
 		{"t / v", "tools / artifacts"},
-		{"S / R", "session stats / resume sessions"},
+		{"S / R", "session stats / resume past sessions"},
 		{"m / g", "swap model / edit config"},
 		{"G / I", "grants / idea board"},
 	})
@@ -842,6 +853,7 @@ func (m Model) helpBody() []string {
 		{"A", "approve-always — choose scope"},
 		{"↑ / ↓", "walk the pending queue"},
 		{"^x", "fullscreen the diff / command preview"},
+		{"esc", "dismiss for now (a re-opens)"},
 	})
 	section("diff / preview viewer (^x)", []kb{
 		{"↑ / ↓", "scroll a line"},