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/getting-started/quickstart.md

@@ -77,11 +77,16 @@ You can switch providers at any time with `hermes model` — no code changes, no
 ## 3. Start Chatting
 
 ```bash
-hermes
+hermes            # classic CLI
+hermes --tui      # modern Ink TUI (recommended)
 ```
 
 That's it! You'll see a welcome banner with your model, available tools, and skills. Type a message and press Enter.
 
+:::tip Pick your interface
+Hermes ships with two terminal interfaces: the classic `prompt_toolkit` CLI and a newer [Ink TUI](../user-guide/tui.md) with modal overlays, mouse selection, and non-blocking input. Both share the same sessions, slash commands, and config — try each with `hermes` vs `hermes --tui`.
+:::
+
 ```
 ❯ What can you help me with?
 ```