feat: add customizable AI prompts with enhanced UX

Add comprehensive prompt customization system allowing users to customize
all AI prompts (Auto Mode, Agent Runner, Backlog Plan, Enhancement) through
the Settings UI.

## Features

### Core Customization System
- New TypeScript types for prompt customization with enabled flag
- CustomPrompt interface with value and enabled state
- Prompts preserved even when disabled (no data loss)
- Merged prompt system (custom overrides defaults when enabled)
- Persistent storage in ~/.automaker/settings.json

### Settings UI
- New "Prompt Customization" section in Settings
- 4 tabs: Auto Mode, Agent, Backlog Plan, Enhancement
- Toggle-based editing (read-only default → editable custom)
- Dynamic textarea height based on prompt length (120px-600px)
- Visual state indicators (Custom/Default labels)

### Warning System
- Critical prompt warnings for Backlog Plan (JSON format requirement)
- Field-level warnings when editing critical prompts
- Info banners for Auto Mode planning markers
- Color-coded warnings (blue=info, amber=critical)

### Backend Integration
- Auto Mode service loads prompts from settings
- Agent service loads prompts from settings
- Backlog Plan service loads prompts from settings
- Enhancement endpoint loads prompts from settings
- Settings sync includes promptCustomization field

### Files Changed
- libs/types/src/prompts.ts - Type definitions
- libs/prompts/src/defaults.ts - Default prompt values
- libs/prompts/src/merge.ts - Merge utilities
- apps/ui/src/components/views/settings-view/prompts/ - UI components
- apps/server/src/lib/settings-helpers.ts - getPromptCustomization()
- All service files updated to use customizable prompts

## Technical Details

Prompt storage format:
```json
{
  "promptCustomization": {
    "autoMode": {
      "planningLite": {
        "value": "Custom prompt text...",
        "enabled": true
      }
    }
  }
}
```

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/ui/src/components/views/settings-view/config/navigation.ts

@@ -10,6 +10,7 @@ import {
   Trash2,
   Sparkles,
   Plug,
+  MessageSquareText,
 } from 'lucide-react';
 import type { SettingsViewId } from '../hooks/use-settings-view';
 
@@ -24,6 +25,7 @@ export const NAV_ITEMS: NavigationItem[] = [
   { id: 'api-keys', label: 'API Keys', icon: Key },
   { id: 'claude', label: 'Claude', icon: Terminal },
   { id: 'mcp-servers', label: 'MCP Servers', icon: Plug },
+  { id: 'prompts', label: 'Prompt Customization', icon: MessageSquareText },
   { id: 'ai-enhancement', label: 'AI Enhancement', icon: Sparkles },
   { id: 'appearance', label: 'Appearance', icon: Palette },
   { id: 'terminal', label: 'Terminal', icon: SquareTerminal },