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/overlays.go

@@ -132,6 +132,8 @@ func (m Model) renderOverlay(base string) string {
 		return m.center(m.grantsModal())
 	case OverlayGrantScope:
 		return m.center(m.grantScopeModal())
+	case OverlayHelp:
+		return m.center(m.helpModal())
 	}
 	return base
 }
@@ -437,8 +439,25 @@ func (m Model) eventInspectorModal() string {
 		b.WriteString(mbg(t, "   ("+itoa(m.overlayEventIdx+1)+"/"+itoa(len(evs))+")", t.P.Faint))
 	}
 	b.WriteString("\n\n")
+	// Filter line (shown while editing or when a query is set): a `/` prompt over the query.
+	if m.eventFilterTyping || m.eventFilter != "" {
+		caret := mbg(t, "", t.P.BgPanel)
+		if m.eventFilterTyping && m.caretVisible() {
+			caret = lipgloss.NewStyle().Foreground(t.P.Accent).Background(t.P.BgPanel).Render("▏")
+		}
+		prompt := lipgloss.NewStyle().Foreground(t.P.Accent).Background(t.P.BgPanel).Render("/ ")
+		if m.eventFilter == "" {
+			b.WriteString(prompt + caret + mbg(t, "type to filter events…", t.P.Faint) + "\n\n")
+		} else {
+			b.WriteString(prompt + mbg(t, m.eventFilter, t.P.FgStrong) + caret + "\n\n")
+		}
+	}
 	if len(evs) == 0 {
-		b.WriteString(mbg(t, "no events", t.P.Faint))
+		msg := "no events"
+		if m.eventFilter != "" {
+			msg = "no events match \"" + m.eventFilter + "\""
+		}
+		b.WriteString(mbg(t, msg, t.P.Faint))
 		return m.center(t.Overlay.Width(w).Render(b.String()))
 	}
 	// Budget the plain detail so the styled row never needs ANSI-aware truncation
@@ -482,7 +501,7 @@ func (m Model) eventInspectorModal() string {
 			mbg(t, " "+truncate(e.Detail, detailBudget), t.P.Dim)
 		b.WriteString(row + "\n")
 	}
-	b.WriteString("\n" + modalHints(t, [][2]string{{"↑↓", "select"}, {"esc", "close"}}))
+	b.WriteString("\n" + modalHints(t, [][2]string{{"↑↓", "select"}, {"/", "filter"}, {"c", "clear"}, {"esc", "close"}}))
 
 	modal := t.Overlay.Width(w).Render(b.String())
 	return m.center(modal)
@@ -688,6 +707,52 @@ func (m Model) grantScopeModal() string {
 	return m.center(modal)
 }
 
+// helpModal is the keybinding cheat-sheet (?), grouped by context. Any key dismisses it.
+func (m Model) helpModal() string {
+	t := m.theme
+	w := m.modalWidth()
+
+	type kb struct{ key, desc string }
+	section := func(b *strings.Builder, title string, rows []kb) {
+		b.WriteString(lipgloss.NewStyle().Foreground(t.P.Accent2).Background(t.P.BgPanel).Bold(true).Render(title) + "\n")
+		for _, r := range rows {
+			key := lipgloss.NewStyle().Foreground(t.P.Accent).Background(t.P.BgPanel).Render(padRaw(r.key, 16))
+			b.WriteString("  " + key + mbg(t, r.desc, t.P.Fg) + "\n")
+		}
+		b.WriteString("\n")
+	}
+
+	var b strings.Builder
+	b.WriteString(m.titleLine("keybindings") + "\n\n")
+	section(&b, "session", []kb{
+		{"i", "compose a message"},
+		{"^↑ / ^↓", "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(&b, "overlays", []kb{
+		{"p", "command palette (shows all shortcuts)"},
+		{"e", "event inspector (/ to filter)"},
+		{"t / v", "tools / artifacts"},
+		{"S / R", "session stats / resume sessions"},
+		{"m / g", "swap model / edit config"},
+		{"G / I", "grants / idea board"},
+	})
+	section(&b, "approval band", []kb{
+		{"y / a / enter", "approve once"},
+		{"n / r", "reject"},
+		{"e / s", "steer (add a note)"},
+		{"A", "approve-always — choose scope"},
+		{"↑ / ↓", "walk the pending queue"},
+	})
+	b.WriteString(modalHints(t, [][2]string{{"any key", "close"}}))
+	modal := t.Overlay.Width(w).Render(b.String())
+	return m.center(modal)
+}
+
 func (m Model) toolPaletteModal() string {
 	t := m.theme
 	w := m.modalWidth()