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/sidebars.ts

@@ -21,6 +21,7 @@ const sidebars: SidebarsConfig = {
       collapsed: true,
       items: [
         'user-guide/cli',
+        'user-guide/tui',
         'user-guide/configuration',
         'user-guide/sessions',
         'user-guide/profiles',