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/model.go

@@ -61,6 +61,7 @@ const (
 	OverlayStatusbar
 	OverlayGrants
 	OverlayGrantScope
+	OverlayHelp
 )
 
 // RouterEntry is one line in a session's conversation transcript.
@@ -327,6 +328,15 @@ type Model struct {
 	// the OUTPUT transcript; they always stay in the EVENTS panel. Persisted in tui-prefs.json.
 	actionsHidden bool
 
+	// outputScroll is how many rows up from the bottom the OUTPUT transcript is scrolled
+	// (0 = tail-follow the newest output). PgUp/PgDn + ctrl+u/d move it; esc snaps back.
+	outputScroll int
+
+	// event-inspector filter (OverlayEventInspector): narrows the event list by a substring
+	// of type/detail. eventFilterTyping is true while the operator is editing the query after /.
+	eventFilter       string
+	eventFilterTyping bool
+
 	// standing grants (OverlayGrants) — active PROJECT/GLOBAL grants from the grant.list reply.
 	grants        []protocol.GrantDto
 	grantsLoading bool