chore(lint): register lint plugin in marketplace and document scope

Adds the lint plugin entry to both marketplace manifests and records
the resolved scope/structure decisions from grilling in CONTEXT.md:
standalone repo-agnostic plugin, split vale-config/vale-run skills,
report-only lint-runner agent, audit-pipeline wiring deferred.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FxG5T8EJDgkABXxuneuFfn

.github/plugin/marketplace.json

@@ -38,7 +38,12 @@
         "repo": "mattpocock/skills",
         "source": "github"
       }
+    },
+    {
+      "description": "Skills and agents for configuring and running linters, starting with Vale.",
+      "name": "lint",
+      "source": "./plugins/lint"
     }
   ],
-  "version": "0.2.0"
+  "version": "0.3.0"
 }