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/server/src/routes/enhance-prompt/index.ts

@@ -6,17 +6,19 @@
  */
 
 import { Router } from 'express';
+import type { SettingsService } from '../../services/settings-service.js';
 import { createEnhanceHandler } from './routes/enhance.js';
 
 /**
  * Create the enhance-prompt router
  *
+ * @param settingsService - Settings service for loading custom prompts
  * @returns Express router with enhance-prompt endpoints
  */
-export function createEnhancePromptRoutes(): Router {
+export function createEnhancePromptRoutes(settingsService?: SettingsService): Router {
   const router = Router();
 
-  router.post('/', createEnhanceHandler());
+  router.post('/', createEnhanceHandler(settingsService));
 
   return router;
 }

apps/server/src/routes/enhance-prompt/routes/enhance.ts

@@ -10,8 +10,9 @@ import { query } from '@anthropic-ai/claude-agent-sdk';
 import { createLogger } from '@automaker/utils';
 import { resolveModelString } from '@automaker/model-resolver';
 import { CLAUDE_MODEL_MAP } from '@automaker/types';
+import type { SettingsService } from '../../../services/settings-service.js';
+import { getPromptCustomization } from '../../../lib/settings-helpers.js';
 import {
-  getSystemPrompt,
   buildUserPrompt,
   isValidEnhancementMode,
   type EnhancementMode,
@@ -83,9 +84,12 @@ async function extractTextFromStream(
 /**
  * Create the enhance request handler
  *
+ * @param settingsService - Optional settings service for loading custom prompts
  * @returns Express request handler for text enhancement
  */
-export function createEnhanceHandler(): (req: Request, res: Response) => Promise<void> {
+export function createEnhanceHandler(
+  settingsService?: SettingsService
+): (req: Request, res: Response) => Promise<void> {
   return async (req: Request, res: Response): Promise<void> => {
     try {
       const { originalText, enhancementMode, model } = req.body as EnhanceRequestBody;
@@ -128,8 +132,20 @@ export function createEnhanceHandler(): (req: Request, res: Response) => Promise
 
       logger.info(`Enhancing text with mode: ${validMode}, length: ${trimmedText.length} chars`);
 
-      // Get the system prompt for this mode
-      const systemPrompt = getSystemPrompt(validMode);
+      // Load enhancement prompts from settings (merges custom + defaults)
+      const prompts = await getPromptCustomization(settingsService, '[EnhancePrompt]');
+
+      // Get the system prompt for this mode from merged prompts
+      const systemPrompt =
+        validMode === 'improve'
+          ? prompts.enhancement.improveSystemPrompt
+          : validMode === 'technical'
+            ? prompts.enhancement.technicalSystemPrompt
+            : validMode === 'simplify'
+              ? prompts.enhancement.simplifySystemPrompt
+              : prompts.enhancement.acceptanceSystemPrompt;
+
+      logger.debug(`Using ${validMode} system prompt (length: ${systemPrompt.length} chars)`);
 
       // Build the user prompt with few-shot examples
       // This helps the model understand this is text transformation, not a coding task