Step 8: TUI — Bubble Tea terminal UI with streaming, tool progress, slash commands

## Context

The TUI is the primary user interface for the HN launch. It connects to the gateway WebSocket and provides a rich terminal experience.

## Scope

### Layout
```
┌─────────────────────────────────────────────────┐
│ Yantra v0.1.0 │ gemini/gemini-2.5-flash │ ● Connected │
├─────────────────────────────────────────────────┤
│ You: │
│ Read the main.go file and explain it │
│ │
│ Yantra: │
│ ◐ Calling read_file... │
│ I've read the main.go file. Here's what... │
├─────────────────────────────────────────────────┤
│ > Type a message... (Ctrl+S send, /help) │ S1 │
└─────────────────────────────────────────────────┘
```

### Components
- **Header bar** — version, provider/model, connection status, session indicator
- **Message viewport** — scrollable chat history with markdown rendering
- User messages: distinct style
- Assistant messages: streaming with cursor (▌)
- Tool calls: collapsible, spinner → result
- Syntax highlighting for code blocks
- **Input area** — multi-line textarea (1-5 lines), Ctrl+S/Enter to send
- **Status bar** — session name, message count, token usage

### Streaming UX
- Tokens appear incrementally with typing cursor
- Tool calls show spinner with tool name
- Tool results collapse into summary line

### Slash commands
| Command | Description |
|---|---|
| `/new [name]` | Create new session |
| `/sessions` | List all sessions |
| `/switch ` | Switch to session |
| `/cancel` | Cancel current turn |
| `/model ` | Switch provider/model |
| `/clear` | Clear display |
| `/help` | Show help |
| `/quit` | Exit |

### WebSocket client
- Connect to gateway `/ws`
- Handle all server frame types
- Reconnect on disconnect

## Files to create
```
internal/tui/
├── app.go # Bubble Tea model + Init/Update/View
├── viewport.go # Message viewport component
├── input.go # Input textarea component
├── header.go # Header bar
├── status.go # Status bar
├── styles.go # Lip Gloss styles
├── ws_client.go # WebSocket client
└── commands.go # Slash command parsing
```

## Dependencies
- `github.com/charmbracelet/bubbletea`
- `github.com/charmbracelet/bubbles`
- `github.com/charmbracelet/lipgloss`
- `github.com/charmbracelet/glamour` (markdown rendering)

## Implements
- `yantra tui` — launches TUI (auto-starts daemon if needed)