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/view.go

@@ -493,10 +493,42 @@ func (m Model) welcomeRows(w int) []string {
 
 func (m Model) routerRows(w, h int) []string {
 	t := m.theme
-	msgs := m.routerMessages[m.selectedID]
-	if len(msgs) == 0 {
+	if len(m.routerMessages[m.selectedID]) == 0 {
 		return []string{t.span("awaiting router response…", t.P.Faint)}
 	}
+	rows, msgStart := m.buildTranscriptRows(w)
+	// With a selection, scroll so the selected message is visible (top-aligned, clamped to the
+	// end); otherwise honour the free-scroll offset (0 = tail-follow the newest output).
+	if m.transcriptSel >= 0 && m.transcriptSel < len(msgStart) && len(rows) > h {
+		start := msgStart[m.transcriptSel]
+		if start > len(rows)-h {
+			start = len(rows) - h
+		}
+		if start < 0 {
+			start = 0
+		}
+		return rows[start : start+h]
+	}
+	if len(rows) > h {
+		off := m.outputScroll
+		if max := len(rows) - h; off > max {
+			off = max
+		}
+		if off < 0 {
+			off = 0
+		}
+		end := len(rows) - off
+		return rows[end-h : end]
+	}
+	return rows
+}
+
+// buildTranscriptRows renders the selected session's transcript to display rows (and records each
+// message's first row index for scroll-to-selection). Split out so the scroll handler can measure
+// the full height against the same content the renderer windows.
+func (m Model) buildTranscriptRows(w int) ([]string, []int) {
+	t := m.theme
+	msgs := m.routerMessages[m.selectedID]
 	var rows []string
 	msgStart := make([]int, len(msgs)) // first row index of each message, for scroll-to-selection
 	for mi, e := range msgs {
@@ -552,22 +584,7 @@ func (m Model) routerRows(w, h int) []string {
 			rows = append(rows, "  "+icon+" "+t.span(e.Content, t.P.Dim))
 		}
 	}
-	// With a selection, scroll so the selected message is visible (top-aligned, clamped
-	// to the end); otherwise tail-follow the newest output.
-	if m.transcriptSel >= 0 && m.transcriptSel < len(msgStart) && len(rows) > h {
-		start := msgStart[m.transcriptSel]
-		if start > len(rows)-h {
-			start = len(rows) - h
-		}
-		if start < 0 {
-			start = 0
-		}
-		return rows[start : start+h]
-	}
-	if len(rows) > h {
-		rows = rows[len(rows)-h:]
-	}
-	return rows
+	return rows, msgStart
 }
 
 func (m Model) eventRows(w, h int) []string {