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/lib/settings-helpers.ts

@@ -4,7 +4,13 @@
 
 import type { SettingsService } from '../services/settings-service.js';
 import type { ContextFilesResult, ContextFileInfo } from '@automaker/utils';
-import type { MCPServerConfig, McpServerConfig } from '@automaker/types';
+import type { MCPServerConfig, McpServerConfig, PromptCustomization } from '@automaker/types';
+import {
+  mergeAutoModePrompts,
+  mergeAgentPrompts,
+  mergeBacklogPlanPrompts,
+  mergeEnhancementPrompts,
+} from '@automaker/prompts';
 
 /**
  * Get the autoLoadClaudeMd setting, with project settings taking precedence over global.
@@ -255,3 +261,43 @@ function convertToSdkFormat(server: MCPServerConfig): McpServerConfig {
     env: server.env,
   };
 }
+
+/**
+ * Get prompt customization from global settings and merge with defaults.
+ * Returns prompts merged with built-in defaults - custom prompts override defaults.
+ *
+ * @param settingsService - Optional settings service instance
+ * @param logPrefix - Prefix for log messages
+ * @returns Promise resolving to merged prompts for all categories
+ */
+export async function getPromptCustomization(
+  settingsService?: SettingsService | null,
+  logPrefix = '[PromptHelper]'
+): Promise<{
+  autoMode: ReturnType<typeof mergeAutoModePrompts>;
+  agent: ReturnType<typeof mergeAgentPrompts>;
+  backlogPlan: ReturnType<typeof mergeBacklogPlanPrompts>;
+  enhancement: ReturnType<typeof mergeEnhancementPrompts>;
+}> {
+  let customization: PromptCustomization = {};
+
+  if (settingsService) {
+    try {
+      const globalSettings = await settingsService.getGlobalSettings();
+      customization = globalSettings.promptCustomization || {};
+      console.log(`${logPrefix} Loaded prompt customization from settings`);
+    } catch (error) {
+      console.error(`${logPrefix} Failed to load prompt customization:`, error);
+      // Fall through to use empty customization (all defaults)
+    }
+  } else {
+    console.log(`${logPrefix} SettingsService not available, using default prompts`);
+  }
+
+  return {
+    autoMode: mergeAutoModePrompts(customization.autoMode),
+    agent: mergeAgentPrompts(customization.agent),
+    backlogPlan: mergeBacklogPlanPrompts(customization.backlogPlan),
+    enhancement: mergeEnhancementPrompts(customization.enhancement),
+  };
+}