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/services/agent-service.ts

@@ -23,6 +23,7 @@ import {
   filterClaudeMdFromContext,
   getMCPServersFromSettings,
   getMCPPermissionSettings,
+  getPromptCustomization,
 } from '../lib/settings-helpers.js';
 
 interface Message {
@@ -75,6 +76,7 @@ export class AgentService {
   private metadataFile: string;
   private events: EventEmitter;
   private settingsService: SettingsService | null = null;
+  private agentSystemPrompt: string | null = null;
 
   constructor(dataDir: string, events: EventEmitter, settingsService?: SettingsService) {
     this.stateDir = path.join(dataDir, 'agent-sessions');
@@ -246,7 +248,7 @@ export class AgentService {
       const contextFilesPrompt = filterClaudeMdFromContext(contextResult, autoLoadClaudeMd);
 
       // Build combined system prompt with base prompt and context files
-      const baseSystemPrompt = this.getSystemPrompt();
+      const baseSystemPrompt = await this.getSystemPrompt();
       const combinedSystemPrompt = contextFilesPrompt
         ? `${contextFilesPrompt}\n\n${baseSystemPrompt}`
         : baseSystemPrompt;
@@ -781,38 +783,13 @@ export class AgentService {
     this.events.emit('agent:stream', { sessionId, ...data });
   }
 
-  private getSystemPrompt(): string {
-    return `You are an AI assistant helping users build software. You are part of the Automaker application,
-which is designed to help developers plan, design, and implement software projects autonomously.
-
-**Feature Storage:**
-Features are stored in .automaker/features/{id}/feature.json - each feature has its own folder.
-Use the UpdateFeatureStatus tool to manage features, not direct file edits.
-
-Your role is to:
-- Help users define their project requirements and specifications
-- Ask clarifying questions to better understand their needs
-- Suggest technical approaches and architectures
-- Guide them through the development process
-- Be conversational and helpful
-- Write, edit, and modify code files as requested
-- Execute commands and tests
-- Search and analyze the codebase
-
-When discussing projects, help users think through:
-- Core functionality and features
-- Technical stack choices
-- Data models and architecture
-- User experience considerations
-- Testing strategies
-
-You have full access to the codebase and can:
-- Read files to understand existing code
-- Write new files
-- Edit existing files
-- Run bash commands
-- Search for code patterns
-- Execute tests and builds`;
+  private async getSystemPrompt(): Promise<string> {
+    // Load from settings if not already cached
+    if (!this.agentSystemPrompt) {
+      const prompts = await getPromptCustomization(this.settingsService, '[AgentService]');
+      this.agentSystemPrompt = prompts.agent.systemPrompt;
+    }
+    return this.agentSystemPrompt;
   }
 
   private generateId(): string {