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

plugins/lint/.mcp.json

@@ -0,0 +1,3 @@
+{
+  "mcpServers": {}
+}