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.

ui-tui/src/domain/paths.ts

@@ -14,27 +14,3 @@ export const fmtCwdBranch = (cwd: string, branch: null | string, max = 40) => {
 
   return `${shortCwd(cwd, Math.max(8, max - tag.length))}${tag}`
 }
-
-/**
- * Compose the terminal titlebar string:
- *   `<marker> <session name> · <model> · <cwd>`
- *
- * The session name and cwd are each omitted when empty, and a long session
- * name is truncated. The marker is always glued to the first present segment
- * with a plain space (not a ` · ` separator). When no model is known yet the
- * caller should fall back to a plain brand string instead of calling this.
- */
-export const composeTabTitle = (
-  marker: string,
-  sessionName: string,
-  model: string,
-  cwd: string,
-  maxName = 28
-): string => {
-  const name = sessionName.trim()
-  const shortName = name.length > maxName ? `${name.slice(0, maxName - 1)}…` : name
-
-  const segments = [shortName, model, cwd].filter(Boolean)
-
-  return segments.length ? `${marker} ${segments.join(' · ')}` : marker
-}