fix: keep plugin.json version in sync with marketplace.json (closes #323) (#326)

Plugin.json's version field had been stuck at 1.9.0 across three releases
(v2.0, v2.0.1, v2.1.0) while marketplace.json was bumped each time. Claude
Code uses plugin.json's version for the update check, so
`claude plugin update marketing-skills@marketingskills` reported "already
at the latest version (1.9.0)" and refused to pull anything new — even
when main had real new content (sms skill, refreshed ai-seo / image / video).

Three changes:

1. .claude-plugin/plugin.json: bumped 1.9.0 → 2.1.0 to clear the current
   drift.

2. .github/scripts/sync-skills.js: added updatePluginVersion() that reads
   marketplace.json's metadata.version and copies it into plugin.json's
   version field. Idempotent — no-op when they already match.

3. .github/workflows/sync-skills.yml: added marketplace.json to the
   `paths` filter so the sync workflow runs whenever marketplace.json
   changes (previously it only ran on skills/** changes, which is why
   release commits that only bumped marketplace.json never triggered the
   sync). Also added .claude-plugin/plugin.json to the file_pattern so
   the bot commit captures the auto-bump.

Verified locally: script bumped 1.9.0 → 2.1.0, then a second run reported
"Everything is already in sync."

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/sync-skills.yml

@@ -5,6 +5,7 @@ on:
     branches: [main]
     paths:
       - 'skills/**'
+      - '.claude-plugin/marketplace.json'
 
 jobs:
   sync:
@@ -26,5 +27,5 @@ jobs:
         with:
           commit_user_name: Coreybot
           commit_user_email: coreybot+github-actions[bot]@users.noreply.github.com
-          commit_message: "chore: sync skills with marketplace.json and README"
-          file_pattern: ".claude-plugin/marketplace.json README.md"
+          commit_message: "chore: sync skills with marketplace.json, plugin.json, and README"
+          file_pattern: ".claude-plugin/marketplace.json .claude-plugin/plugin.json README.md"