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/store/app-store.ts

@@ -11,6 +11,7 @@ import type {
   FeatureStatusWithPipeline,
   PipelineConfig,
   PipelineStep,
+  PromptCustomization,
 } from '@automaker/types';
 
 // Re-export ThemeMode for convenience
@@ -492,6 +493,9 @@ export interface AppState {
   mcpAutoApproveTools: boolean; // Auto-approve MCP tool calls without permission prompts
   mcpUnrestrictedTools: boolean; // Allow unrestricted tools when MCP servers are enabled
 
+  // Prompt Customization
+  promptCustomization: PromptCustomization; // Custom prompts for Auto Mode, Agent, Backlog Plan, Enhancement
+
   // Project Analysis
   projectAnalysis: ProjectAnalysis | null;
   isAnalyzing: boolean;
@@ -774,6 +778,9 @@ export interface AppActions {
   setMcpAutoApproveTools: (enabled: boolean) => Promise<void>;
   setMcpUnrestrictedTools: (enabled: boolean) => Promise<void>;
 
+  // Prompt Customization actions
+  setPromptCustomization: (customization: PromptCustomization) => Promise<void>;
+
   // AI Profile actions
   addAIProfile: (profile: Omit<AIProfile, 'id'>) => void;
   updateAIProfile: (id: string, updates: Partial<AIProfile>) => void;
@@ -972,6 +979,7 @@ const initialState: AppState = {
   mcpServers: [], // No MCP servers configured by default
   mcpAutoApproveTools: true, // Default to enabled - bypass permission prompts for MCP tools
   mcpUnrestrictedTools: true, // Default to enabled - don't filter allowedTools when MCP enabled
+  promptCustomization: {}, // Empty by default - all prompts use built-in defaults
   aiProfiles: DEFAULT_AI_PROFILES,
   projectAnalysis: null,
   isAnalyzing: false,
@@ -1628,6 +1636,14 @@ export const useAppStore = create<AppState & AppActions>()(
         await syncSettingsToServer();
       },
 
+      // Prompt Customization actions
+      setPromptCustomization: async (customization) => {
+        set({ promptCustomization: customization });
+        // Sync to server settings file
+        const { syncSettingsToServer } = await import('@/hooks/use-settings-migration');
+        await syncSettingsToServer();
+      },
+
       // AI Profile actions
       addAIProfile: (profile) => {
         const id = `profile-${Date.now()}-${Math.random().toString(36).substr(2, 9)}`;
@@ -2909,6 +2925,8 @@ export const useAppStore = create<AppState & AppActions>()(
           mcpServers: state.mcpServers,
           mcpAutoApproveTools: state.mcpAutoApproveTools,
           mcpUnrestrictedTools: state.mcpUnrestrictedTools,
+          // Prompt customization
+          promptCustomization: state.promptCustomization,
           // Profiles and sessions
           aiProfiles: state.aiProfiles,
           chatSessions: state.chatSessions,