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/cli-commands.md

@@ -27,6 +27,8 @@ hermes [global-options] <command> [subcommand/options]
 | `--worktree`, `-w` | Start in an isolated git worktree for parallel-agent workflows. |
 | `--yolo` | Bypass dangerous-command approval prompts. |
 | `--pass-session-id` | Include the session ID in the agent's system prompt. |
+| `--tui` | Launch the [Ink TUI](../user-guide/tui.md) instead of the classic CLI. Equivalent to `HERMES_TUI=1`. |
+| `--dev` | With `--tui`: run the TypeScript sources directly via `tsx` instead of the prebuilt bundle (for TUI contributors). |
 
 ## Top-level commands