opentui(phase3): launcher integration — HERMES_TUI_ENGINE dual-engine

hermes --tui launches the native OpenTUI engine (Bun) when
HERMES_TUI_ENGINE=opentui (env) or display.tui_engine=opentui (config);
Ink stays the default and the shipping path is untouched.

- _resolve_tui_engine() (env > config > ink); refuses opentui on
  Windows/Termux (no Bun) -> falls back to ink with a notice.
- _make_opentui_argv() -> [bun, src/entry.real.tsx] (no build step).
- _bun_bin() with HERMES_BUN override.
- Branch at top of _make_tui_argv BEFORE _ensure_tui_node (Bun-only host
  must not bootstrap Node).
- Gate _launch_tui NODE_OPTIONS/--max-old-space-size on engine==ink (Bun
  is JSC; the V8 flag errors/ignores).

Verified end-to-end via tmux: real hermes --tui -> Bun -> OpenTUI ->
real Python gateway streamed a real reply. No-flag default still ink.

apps/desktop/electron/session-windows.cjs

@@ -1,86 +0,0 @@
-// Secondary "session windows" — one extra OS window per chat so a user can
-// work with multiple chats side by side. The pure, Electron-free pieces live
-// here so they can be unit-tested with node --test (mirroring how the rest of
-// electron/*.cjs splits testable logic out of the main.cjs monolith).
-
-const { pathToFileURL } = require('node:url')
-
-// Build the renderer URL for a secondary window. The renderer uses a
-// HashRouter, so the session route lives after the '#'. The `?win=secondary`
-// flag MUST sit in the query string BEFORE the '#': anything after the '#' is
-// treated as the route by HashRouter and would break routeSessionId(). The
-// renderer reads the flag from window.location.search to suppress the install /
-// onboarding overlays and the global session sidebar.
-function buildSessionWindowUrl(sessionId, { devServer, rendererIndexPath } = {}) {
-  const route = `#/${encodeURIComponent(sessionId)}`
-
-  if (devServer) {
-    const base = devServer.endsWith('/') ? devServer.slice(0, -1) : devServer
-
-    return `${base}/?win=secondary${route}`
-  }
-
-  return `${pathToFileURL(rendererIndexPath).toString()}?win=secondary${route}`
-}
-
-// A small registry keyed by sessionId that guarantees one window per chat:
-// opening a session that already has a live window focuses it instead of
-// spawning a duplicate, and a window removes itself from the registry when it
-// closes. The actual BrowserWindow construction is injected (the `factory`) so
-// this module stays free of Electron and is unit-testable.
-function createSessionWindowRegistry() {
-  const windows = new Map()
-
-  function openOrFocus(sessionId, factory) {
-    const key = typeof sessionId === 'string' ? sessionId.trim() : ''
-
-    if (!key) {
-      return null
-    }
-
-    const existing = windows.get(key)
-
-    if (existing && !existing.isDestroyed()) {
-      // Focus-or-create: never duplicate a window for the same chat.
-      if (typeof existing.isMinimized === 'function' && existing.isMinimized()) {
-        existing.restore?.()
-      }
-
-      if (typeof existing.isVisible === 'function' && !existing.isVisible()) {
-        existing.show?.()
-      }
-
-      existing.focus?.()
-
-      return existing
-    }
-
-    const win = factory(key)
-
-    if (!win) {
-      return null
-    }
-
-    windows.set(key, win)
-
-    // Self-cleanup on close so the registry never holds a destroyed window.
-    win.on?.('closed', () => {
-      if (windows.get(key) === win) {
-        windows.delete(key)
-      }
-    })
-
-    return win
-  }
-
-  return {
-    openOrFocus,
-    get: key => windows.get(key),
-    has: key => windows.has(key),
-    get size() {
-      return windows.size
-    }
-  }
-}
-
-module.exports = { buildSessionWindowUrl, createSessionWindowRegistry }