fix(terminal): share the gesture-aware page-key math with tiles, add opencode

Review follow-up after the rebase onto 1.40.0, where tiles began paging a
hollow buffer through the shared CodemanTerminalInput helpers and opencode
joined the hollow check (#555).

- opencode declares transcriptPageKeys in stock.ts, so the paging list read
  by both panes is claude, codex and opencode. Without it, reading the
  capability instead of the old id check would have turned opencode paging
  off again. The stock-set and render tests name it.
- The gesture rules (first trackpad-sized event pages at once and is owed
  back, wheel notches accumulate, a pre-paid debt dies with its gesture,
  pinch and mostly horizontal swipes are consumed without paging) move out
  of the primary pane into the pure CodemanTerminalInput.pageKeysForGesture.
  The primary pane and TerminalTile both call it with their own state, so a
  Codex or opencode tile pages exactly like the main pane.
- The dialog check stays per pane: the primary pane reads tabAlerts for the
  active session, a tile for its own sessionId, so a wheel over a tile never
  pages into that session's permission prompt while the focused tab is clear,
  and a tile is not blocked by a dialog in another tab.

Tests: pure helper cases in terminal-scroll-routing, tile cases for Codex
paging, first-event paging, notch accumulation, pinch and swipe, byte parity
with the primary pane over a mixed sequence, and the per-session dialog
check in both directions.

src/config/cli-registry/stock.ts

@@ -497,6 +497,11 @@ const OPENCODE: CliEntry = {
     ...agentDefaults(),
     altScreen: 'strip-mux-and-mouse',
     echo: { policy: 'buffer', anchor: { kind: 'cursor' }, predictProfile: undefined },
+    // Pages its own transcript on PageUp/PageDown (`messages_page_up/down`) and ignores
+    // SGR wheel reports, and its TUI runs on the alternate screen (1.18.31: tmux
+    // `alternate_on=1`, `history_size=0`), so a pane keeps no local history and the
+    // wheel is turned into page keys or it does nothing at all (#555).
+    transcriptPageKeys: true,
     // Measured on a live opencode 1.3.0 pane (capture-pane every 250-300 ms through real
     // turns at 40, 60, 120 and 200 columns, plus the raw PTY stream, 2026-10-09). Every
     // composer row starts with a `┃` bar, and the submitted prompt lands in the transcript