docs(tui): add Ink TUI user guide + cross-link from CLI docs

New primary guide at `user-guide/tui.md` covering launch, requirements,
keybindings, slash commands, status line, configuration, sessions, and
the revert path. Matches the voice of `user-guide/cli.md`.

Cross-links:
- `user-guide/cli.md`: tip callout pointing readers at the Ink TUI
- `getting-started/quickstart.md`: shows both `hermes` and `hermes --tui`
  under "Start Chatting" so first-run users know they have the choice
- `reference/environment-variables.md`: new "Interface" section with
  `HERMES_TUI` and `HERMES_TUI_DIR`
- `reference/cli-commands.md`: `--tui` and `--dev` added to global options

Sidebar: `user-guide/tui` slotted right after `user-guide/cli`.

website/docs/reference/environment-variables.md

@@ -342,6 +342,13 @@ For cloud sandbox backends, persistence is filesystem-oriented. `TERMINAL_LIFETI
 | `HERMES_BACKGROUND_NOTIFICATIONS` | Background process notification mode in gateway: `all` (default), `result`, `error`, `off` |
 | `HERMES_EPHEMERAL_SYSTEM_PROMPT` | Ephemeral system prompt injected at API-call time (never persisted to sessions) |
 
+## Interface
+
+| Variable | Description |
+|----------|-------------|
+| `HERMES_TUI` | Launch the [Ink TUI](../user-guide/tui.md) instead of the classic CLI when set to `1`. Equivalent to passing `--tui`. |
+| `HERMES_TUI_DIR` | Path to a prebuilt `ui-tui/` directory (must contain `dist/entry.js` and populated `node_modules`). Used by distros and Nix to skip the first-launch `npm install`. |
+
 ## Cron Scheduler
 
 | Variable | Description |