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.tsx

@@ -19,6 +19,7 @@ import { KeyboardShortcutsSection } from './settings-view/keyboard-shortcuts/key
 import { FeatureDefaultsSection } from './settings-view/feature-defaults/feature-defaults-section';
 import { DangerZoneSection } from './settings-view/danger-zone/danger-zone-section';
 import { MCPServersSection } from './settings-view/mcp-servers';
+import { PromptCustomizationSection } from './settings-view/prompts';
 import type { Project as SettingsProject, Theme } from './settings-view/shared/types';
 import type { Project as ElectronProject } from '@/lib/electron';
 
@@ -53,6 +54,8 @@ export function SettingsView() {
     setAutoLoadClaudeMd,
     enableSandboxMode,
     setEnableSandboxMode,
+    promptCustomization,
+    setPromptCustomization,
   } = useAppStore();
 
   // Hide usage tracking when using API key (only show for Claude Code CLI users)
@@ -119,6 +122,13 @@ export function SettingsView() {
         );
       case 'mcp-servers':
         return <MCPServersSection />;
+      case 'prompts':
+        return (
+          <PromptCustomizationSection
+            promptCustomization={promptCustomization}
+            onPromptCustomizationChange={setPromptCustomization}
+          />
+        );
       case 'ai-enhancement':
         return <AIEnhancementSection />;
       case 'appearance':