chore: set up changesets

Add changesets for tracking and versioning skill changes:
- package.json (private) + @changesets/cli and @changesets/changelog-github
- .changeset/config.json using the GitHub release-style changelog
- release workflow that opens a version PR on push to main (no npm publish)
- initial changeset recording recent skill updates

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.changeset/config.json

@@ -0,0 +1,14 @@
+{
+  "$schema": "https://unpkg.com/@changesets/config@3.1.4/schema.json",
+  "changelog": [
+    "@changesets/changelog-github",
+    { "repo": "mattpocock/skills" }
+  ],
+  "commit": false,
+  "fixed": [],
+  "linked": [],
+  "access": "restricted",
+  "baseBranch": "main",
+  "updateInternalDependencies": "patch",
+  "ignore": []
+}