fix(plugin): restore marketplace plugin version

`tests/test_plugin_contract.py::test_versions_match_across_manifests`
enforces that every version-bearing surface agrees: pyproject.toml,
SKILL.md, both plugin.json files, AND the marketplace plugin entry.
The Claude Code spec says plugin.json wins when both are set, but this
repo deliberately mirrors the version across all surfaces and tests it.
Restore the field at 3.1.1 to satisfy the contract.

.claude-plugin/marketplace.json

@@ -11,6 +11,7 @@
     {
       "name": "last30days",
       "description": "Research any topic across Reddit, X, YouTube, TikTok, Instagram, Hacker News, Polymarket, GitHub, and 5+ more sources. AI agent scores by upvotes, likes, and real money - not editors.",
+      "version": "3.1.1",
       "author": {
         "name": "Matt Van Horn",
         "url": "https://github.com/mvanhorn"