refactor: split types.ts into domain modules (phase 4, step 1)

Split 1,443-line types.ts into 14 focused domain files under src/types/:
common.ts, session.ts, task.ts, app-state.ts, respawn.ts, ralph.ts,
api.ts, lifecycle.ts, run-summary.ts, tools.ts, teams.ts, push.ts,
plan.ts, and index.ts barrel.

Moved PlanItem interface from plan-orchestrator.ts into types/plan.ts
to break circular dependency. Original types.ts replaced with barrel
re-export — zero changes to 36 import sites.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/types/api.ts

@@ -0,0 +1,136 @@
+/**
+ * @fileoverview API types and error handling
+ */
+
+/**
+ * Standard error codes for API responses
+ */
+export enum ApiErrorCode {
+  /** Resource not found */
+  NOT_FOUND = 'NOT_FOUND',
+  /** Invalid input provided */
+  INVALID_INPUT = 'INVALID_INPUT',
+  /** Session is currently busy */
+  SESSION_BUSY = 'SESSION_BUSY',
+  /** Operation failed */
+  OPERATION_FAILED = 'OPERATION_FAILED',
+  /** Resource already exists */
+  ALREADY_EXISTS = 'ALREADY_EXISTS',
+  /** Internal server error */
+  INTERNAL_ERROR = 'INTERNAL_ERROR',
+}
+
+/**
+ * User-friendly error messages for each error code
+ */
+const ErrorMessages: Record<ApiErrorCode, string> = {
+  [ApiErrorCode.NOT_FOUND]: 'The requested resource was not found',
+  [ApiErrorCode.INVALID_INPUT]: 'Invalid input provided',
+  [ApiErrorCode.SESSION_BUSY]: 'Session is currently busy',
+  [ApiErrorCode.OPERATION_FAILED]: 'The operation failed',
+  [ApiErrorCode.ALREADY_EXISTS]: 'Resource already exists',
+  [ApiErrorCode.INTERNAL_ERROR]: 'An internal error occurred',
+};
+
+/**
+ * Hook event types triggered by Claude Code's hooks system
+ */
+export type HookEventType =
+  | 'idle_prompt'
+  | 'permission_prompt'
+  | 'elicitation_dialog'
+  | 'stop'
+  | 'teammate_idle'
+  | 'task_completed';
+
+// ========== API Response Types ==========
+
+/**
+ * Standard API response wrapper (discriminated union for type safety)
+ * @template T Type of the data payload
+ */
+export type ApiResponse<T = unknown> =
+  | { success: true; data?: T }
+  | { success: false; error: string; errorCode: ApiErrorCode };
+
+/**
+ * Creates a standardized error response
+ * @param code Error code
+ * @param details Optional detailed error message
+ * @returns Formatted error response
+ */
+export function createErrorResponse(code: ApiErrorCode, details?: string): ApiResponse<never> {
+  return {
+    success: false,
+    error: details || ErrorMessages[code],
+    errorCode: code,
+  };
+}
+
+/**
+ * Response for quick start operation
+ */
+export interface QuickStartResponse {
+  /** Whether the request succeeded */
+  success: boolean;
+  /** Created session ID */
+  sessionId?: string;
+  /** Path to case folder */
+  casePath?: string;
+  /** Case name */
+  caseName?: string;
+  /** Error message if failed */
+  error?: string;
+}
+
+/**
+ * Information about a case folder
+ */
+export interface CaseInfo {
+  /** Case name */
+  name: string;
+  /** Full path to case folder */
+  path: string;
+  /** Whether CLAUDE.md exists */
+  hasClaudeMd?: boolean;
+}
+
+// ========== Error Handling Utilities ==========
+
+/**
+ * Type guard to check if a value is an Error instance
+ * @param value The value to check
+ * @returns True if the value is an Error instance
+ */
+export function isError(value: unknown): value is Error {
+  return value instanceof Error;
+}
+
+/**
+ * Safely extracts an error message from an unknown caught value.
+ * Handles the TypeScript 4.4+ unknown error type in catch blocks.
+ *
+ * @param error The caught error (type unknown in strict mode)
+ * @returns A string error message
+ *
+ * @example
+ * ```typescript
+ * try {
+ *   await riskyOperation();
+ * } catch (err) {
+ *   console.error('Failed:', getErrorMessage(err));
+ * }
+ * ```
+ */
+export function getErrorMessage(error: unknown): string {
+  if (isError(error)) {
+    return error.message;
+  }
+  if (typeof error === 'string') {
+    return error;
+  }
+  if (error && typeof error === 'object' && 'message' in error) {
+    return String((error as { message: unknown }).message);
+  }
+  return 'An unknown error occurred';
+}