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>

scripts/update-core.ts

@@ -1,34 +1,59 @@
 #!/usr/bin/env tsx
 import { applyUpdate, previewUpdate } from '../skills-engine/update.js';
 
-const newCorePath = process.argv[2];
+const args = process.argv.slice(2);
+const jsonMode = args.includes('--json');
+const previewOnly = args.includes('--preview-only');
+const newCorePath = args.find((a) => !a.startsWith('--'));
+
 if (!newCorePath) {
-  console.error('Usage: tsx scripts/update-core.ts <path-to-new-core>');
+  console.error(
+    'Usage: tsx scripts/update-core.ts [--json] [--preview-only] <path-to-new-core>',
+  );
   process.exit(1);
 }
 
 // Preview
 const preview = previewUpdate(newCorePath);
-console.log('=== Update Preview ===');
-console.log(`Current version: ${preview.currentVersion}`);
-console.log(`New version:     ${preview.newVersion}`);
-console.log(`Files changed:   ${preview.filesChanged.length}`);
-if (preview.filesChanged.length > 0) {
-  for (const f of preview.filesChanged) {
-    console.log(`  ${f}`);
+
+if (jsonMode && previewOnly) {
+  console.log(JSON.stringify(preview, null, 2));
+  process.exit(0);
+}
+
+function printPreview(): void {
+  console.log('=== Update Preview ===');
+  console.log(`Current version: ${preview.currentVersion}`);
+  console.log(`New version:     ${preview.newVersion}`);
+  console.log(`Files changed:   ${preview.filesChanged.length}`);
+  if (preview.filesChanged.length > 0) {
+    for (const f of preview.filesChanged) {
+      console.log(`  ${f}`);
+    }
+  }
+  if (preview.conflictRisk.length > 0) {
+    console.log(`Conflict risk:   ${preview.conflictRisk.join(', ')}`);
+  }
+  if (preview.customPatchesAtRisk.length > 0) {
+    console.log(
+      `Custom patches at risk: ${preview.customPatchesAtRisk.join(', ')}`,
+    );
   }
 }
-if (preview.conflictRisk.length > 0) {
-  console.log(`Conflict risk:   ${preview.conflictRisk.join(', ')}`);
-}
-if (preview.customPatchesAtRisk.length > 0) {
-  console.log(`Custom patches at risk: ${preview.customPatchesAtRisk.join(', ')}`);
-}
-console.log('');
 
-// Apply
-console.log('Applying update...');
+if (previewOnly) {
+  printPreview();
+  process.exit(0);
+}
+
+if (!jsonMode) {
+  printPreview();
+  console.log('');
+  console.log('Applying update...');
+}
+
 const result = await applyUpdate(newCorePath);
+
 console.log(JSON.stringify(result, null, 2));
 
 if (!result.success) {