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>

libs/types/src/index.ts

@@ -49,6 +49,21 @@ export { specOutputSchema } from './spec.js';
 // Enhancement types
 export type { EnhancementMode, EnhancementExample } from './enhancement.js';
 
+// Prompt customization types
+export type {
+  CustomPrompt,
+  AutoModePrompts,
+  AgentPrompts,
+  BacklogPlanPrompts,
+  EnhancementPrompts,
+  PromptCustomization,
+  ResolvedAutoModePrompts,
+  ResolvedAgentPrompts,
+  ResolvedBacklogPlanPrompts,
+  ResolvedEnhancementPrompts,
+} from './prompts.js';
+export { DEFAULT_PROMPT_CUSTOMIZATION } from './prompts.js';
+
 // Settings types and constants
 export type {
   ThemeMode,

libs/types/src/prompts.ts

@@ -0,0 +1,153 @@
+/**
+ * Prompt Customization Types
+ *
+ * Defines the structure for customizable AI prompts used throughout the application.
+ * Allows users to modify prompts for Auto Mode, Agent Runner, and Backlog Planning.
+ */
+
+/**
+ * CustomPrompt - A custom prompt with its value and enabled state
+ *
+ * The value is always preserved even when disabled, so users don't lose their work.
+ */
+export interface CustomPrompt {
+  /** The custom prompt text */
+  value: string;
+
+  /** Whether this custom prompt should be used (when false, default is used instead) */
+  enabled: boolean;
+}
+
+/**
+ * AutoModePrompts - Customizable prompts for Auto Mode feature implementation
+ *
+ * Controls how the AI plans and implements features in autonomous mode.
+ */
+export interface AutoModePrompts {
+  /** Planning mode: Quick outline without approval (lite mode) */
+  planningLite?: CustomPrompt;
+
+  /** Planning mode: Quick outline with approval required (lite with approval) */
+  planningLiteWithApproval?: CustomPrompt;
+
+  /** Planning mode: Detailed specification with task breakdown (spec mode) */
+  planningSpec?: CustomPrompt;
+
+  /** Planning mode: Comprehensive Software Design Document (full SDD mode) */
+  planningFull?: CustomPrompt;
+
+  /** Template for building feature implementation prompts */
+  featurePromptTemplate?: CustomPrompt;
+
+  /** Template for follow-up prompts when resuming work */
+  followUpPromptTemplate?: CustomPrompt;
+
+  /** Template for continuation prompts */
+  continuationPromptTemplate?: CustomPrompt;
+
+  /** Template for pipeline step execution prompts */
+  pipelineStepPromptTemplate?: CustomPrompt;
+}
+
+/**
+ * AgentPrompts - Customizable prompts for Agent Runner (chat mode)
+ *
+ * Controls the AI's behavior in interactive chat sessions.
+ */
+export interface AgentPrompts {
+  /** System prompt defining the agent's role and behavior in chat */
+  systemPrompt?: CustomPrompt;
+}
+
+/**
+ * BacklogPlanPrompts - Customizable prompts for Kanban board planning
+ *
+ * Controls how the AI modifies the feature backlog via the Plan button.
+ */
+export interface BacklogPlanPrompts {
+  /** System prompt for backlog plan generation (defines output format and rules) */
+  systemPrompt?: CustomPrompt;
+
+  /** Template for user prompt (includes current features and user request) */
+  userPromptTemplate?: CustomPrompt;
+}
+
+/**
+ * EnhancementPrompts - Customizable prompts for feature description enhancement
+ *
+ * Controls how the AI enhances feature titles and descriptions.
+ */
+export interface EnhancementPrompts {
+  /** System prompt for "improve" mode (vague → clear) */
+  improveSystemPrompt?: CustomPrompt;
+
+  /** System prompt for "technical" mode (add technical details) */
+  technicalSystemPrompt?: CustomPrompt;
+
+  /** System prompt for "simplify" mode (verbose → concise) */
+  simplifySystemPrompt?: CustomPrompt;
+
+  /** System prompt for "acceptance" mode (add acceptance criteria) */
+  acceptanceSystemPrompt?: CustomPrompt;
+}
+
+/**
+ * PromptCustomization - Complete set of customizable prompts
+ *
+ * All fields are optional. Undefined values fall back to built-in defaults.
+ * Stored in GlobalSettings to allow user customization.
+ */
+export interface PromptCustomization {
+  /** Auto Mode prompts (feature implementation) */
+  autoMode?: AutoModePrompts;
+
+  /** Agent Runner prompts (interactive chat) */
+  agent?: AgentPrompts;
+
+  /** Backlog planning prompts (Plan button) */
+  backlogPlan?: BacklogPlanPrompts;
+
+  /** Enhancement prompts (feature description improvement) */
+  enhancement?: EnhancementPrompts;
+}
+
+/**
+ * Default empty prompt customization (all undefined → use built-in defaults)
+ */
+export const DEFAULT_PROMPT_CUSTOMIZATION: PromptCustomization = {
+  autoMode: {},
+  agent: {},
+  backlogPlan: {},
+  enhancement: {},
+};
+
+/**
+ * Resolved prompt types - all fields are required strings (ready to use)
+ * Used for default prompts and merged prompts after resolving custom values
+ */
+export interface ResolvedAutoModePrompts {
+  planningLite: string;
+  planningLiteWithApproval: string;
+  planningSpec: string;
+  planningFull: string;
+  featurePromptTemplate: string;
+  followUpPromptTemplate: string;
+  continuationPromptTemplate: string;
+  pipelineStepPromptTemplate: string;
+}
+
+export interface ResolvedAgentPrompts {
+  systemPrompt: string;
+}
+
+export interface ResolvedBacklogPlanPrompts {
+  systemPrompt: string;
+  userPromptTemplate: string;
+}
+
+export interface ResolvedEnhancementPrompts {
+  improveSystemPrompt: string;
+  technicalSystemPrompt: string;
+  simplifySystemPrompt: string;
+  acceptanceSystemPrompt: string;
+}

libs/types/src/settings.ts

@@ -7,6 +7,7 @@
  */
 
 import type { AgentModel } from './model.js';
+import type { PromptCustomization } from './prompts.js';
 
 // Re-export AgentModel for convenience
 export type { AgentModel };
@@ -360,6 +361,10 @@ export interface GlobalSettings {
   mcpAutoApproveTools?: boolean;
   /** Allow unrestricted tools when MCP servers are enabled (don't filter allowedTools) */
   mcpUnrestrictedTools?: boolean;
+
+  // Prompt Customization
+  /** Custom prompts for Auto Mode, Agent Runner, Backlog Planning, and Enhancements */
+  promptCustomization?: PromptCustomization;
 }
 
 /**