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.

website/docs/user-guide/desktop.md

@@ -233,26 +233,6 @@ rm -rf "$HOME/.hermes/hermes-agent/venv"
 tccutil reset Microphone com.nousresearch.hermes
 ```
 
-### "Build desktop app" stuck on Electron download
-
-The build downloads the Electron runtime (~114 MB) from `github.com/electron/electron/releases`. If the installer hangs on the **Build desktop app** step with the live output repeating `retrying attempt=…`, GitHub is being blocked or throttled on your network (firewall, proxy, or region).
-
-The installer self-heals this automatically: on a failed build it (1) clears a corrupt cached Electron zip and retries, then (2) if it still fails and you haven't set `ELECTRON_MIRROR`, retries once more through `npmmirror.com`, the de-facto Electron community mirror. `@electron/get` SHASUM-checks the download, but the checksums come from the same mirror — that catches a corrupt or partial download, not a compromised mirror. If you'd rather not trust a third-party host, pin your own `ELECTRON_MIRROR` (below); the build never overrides one you've set.
-
-To **choose your own mirror** (e.g. a corporate/trusted one), set `ELECTRON_MIRROR` before installing or rebuild manually — the build honors it and won't override it:
-
-```bash
-ELECTRON_MIRROR=https://npmmirror.com/mirrors/electron/ \
-  bash -c 'cd "$HOME/.hermes/hermes-agent/apps/desktop" && CSC_IDENTITY_AUTO_DISCOVERY=false npm run pack'
-```
-
-To clear a corrupt cached zip by hand:
-
-```bash
-rm -f "$HOME/Library/Caches/electron"/electron-*.zip   # macOS
-rm -f "$HOME/.cache/electron"/electron-*.zip            # Linux
-```
-
 ## Building from source
 
 If you want to hack on the app itself, install workspace deps from the repo root once, then run the dev server from `apps/desktop`: