feat: add /update skill for pulling upstream changes (#372)

Interactive skill that guides Claude through fetching upstream NanoClaw,
previewing changes, merging with customizations, running migrations, and
verifying the result. Includes:

- SKILL.md with 9-step update flow
- fetch-upstream.sh: detects remote, fetches, extracts tracked paths
- run-migrations.ts: discovers and runs version-ordered migrations
- post-update.ts: clears backup after conflict resolution
- update-core.ts: adds --json and --preview-only flags
- BASE_INCLUDES moved to constants.ts as single source of truth
- 16 new tests covering fetch, migrations, and CLI flags

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

skills-engine/init.ts

@@ -2,14 +2,11 @@ import { execSync } from 'child_process';
 import fs from 'fs';
 import path from 'path';
 
-import { BACKUP_DIR, BASE_DIR, NANOCLAW_DIR } from './constants.js';
+import { BACKUP_DIR, BASE_DIR, BASE_INCLUDES, NANOCLAW_DIR } from './constants.js';
 import { isGitRepo } from './merge.js';
 import { writeState } from './state.js';
 import { SkillState } from './types.js';
 
-// Top-level paths to include in base snapshot
-const BASE_INCLUDES = ['src/', 'package.json', '.env.example', 'container/'];
-
 // Directories/files to always exclude from base snapshot
 const BASE_EXCLUDES = [
   'node_modules',