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/constants.ts

@@ -7,3 +7,7 @@ export const CUSTOM_DIR = '.nanoclaw/custom';
 export const RESOLUTIONS_DIR = '.nanoclaw/resolutions';
 export const SHIPPED_RESOLUTIONS_DIR = '.claude/resolutions';
 export const SKILLS_SCHEMA_VERSION = '0.1.0';
+
+// Top-level paths to include in base snapshot and upstream extraction.
+// Add new entries here when new root-level directories/files need tracking.
+export const BASE_INCLUDES = ['src/', 'package.json', '.env.example', 'container/'];