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/git-root.cjs

@@ -1,54 +0,0 @@
-'use strict'
-
-const fs = require('node:fs')
-const path = require('node:path')
-const { resolveRequestedPathForIpc } = require('./hardening.cjs')
-
-function findGitRoot(start, fsImpl = fs) {
-  let dir = start
-
-  for (let i = 0; i < 50; i += 1) {
-    try {
-      if (fsImpl.existsSync(path.join(dir, '.git'))) {
-        return dir
-      }
-    } catch {
-      return null
-    }
-
-    const parent = path.dirname(dir)
-
-    if (parent === dir) {
-      return null
-    }
-
-    dir = parent
-  }
-
-  return null
-}
-
-async function gitRootForIpc(startPath, options = {}) {
-  const fsImpl = options.fs || fs
-  let resolved
-
-  try {
-    resolved = resolveRequestedPathForIpc(startPath, { purpose: 'Git root' })
-  } catch {
-    return null
-  }
-
-  try {
-    const stat = await fsImpl.promises.stat(resolved)
-    const start = stat.isDirectory() ? resolved : path.dirname(resolved)
-
-    return findGitRoot(start, fsImpl)
-  } catch {
-    return findGitRoot(resolved, fsImpl)
-  }
-}
-
-module.exports = {
-  findGitRoot,
-  gitRootForIpc
-}