feat(tui-go): transcript free-scroll + help overlay + event filter

Three TUI-polish items.

- OUTPUT free-scroll: PgUp/PgDn (and ^u/^d half-page) scroll the transcript
  back through history; esc snaps to tail-follow. outputScroll is clamped
  against outputViewport(), which mirrors View/renderMain geometry, so the
  edges land exactly (no dead presses unwinding an overshoot). routerRows
  split into buildTranscriptRows (full render) + windowing so the handler
  measures the same content. Tests cover the clamp + the fits-in-panel no-op.
- Help overlay: `?` (or the "help" palette command) opens a keybinding
  cheat-sheet grouped by context (session / overlays / approval band) — the
  non-palette keys especially. Any key dismisses it.
- Event-inspector filter: `/` filters the event list by a case-insensitive
  substring of type/detail (currentEvents applies it); `c` clears, esc stops
  editing then closes. Fresh per open. EVENTS side panel stays unfiltered.

go build / vet / test green; rendered help + filtered inspector via
cmd/preview (help, events-filter kinds).

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

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

@@ -184,6 +184,27 @@ func PreviewFrame(kind string, w, h int) string {
 			s.LastEventAt = nowMillis() - 3000
 		}
 
+	case "help":
+		m.connected = true
+		m.currentModel = "llama-cpp:default"
+		m.sessions = sampleSessions()
+		m.selectedID = "04a546aa"
+		m.sessionEntered = true
+		m.overlay = OverlayHelp
+
+	case "events-filter":
+		m.connected = true
+		m.currentModel = "llama-cpp:default"
+		m.sessions = sampleSessions()
+		m.selectedID = "04a546aa"
+		m.sessionEntered = true
+		if s := m.session("04a546aa"); s != nil {
+			s.Events = sampleEvents()
+		}
+		m.overlay = OverlayEventInspector
+		m.eventFilter = "tool"
+		m.eventFilterTyping = true
+
 	case "grants":
 		m.connected = true
 		m.currentModel = "llama-cpp:default"