fix(llm-security): correct distribution URLs to marketplace path

The plugin lives in ktg-plugin-marketplace and is distributed via the
Claude Code marketplace mechanism. There is no standalone
open/claude-code-llm-security repo; references to it were aspirational
and never realized.

- package.json: homepage now deep-links to plugins/llm-security/ in the
  marketplace; repository.url uses the marketplace repo with directory
  field (npm convention for monorepo plugins); bugs.url routes to
  marketplace issue tracker.
- CLAUDE.md: "Public Repository" section replaced with "Distribution"
  section documenting the marketplace install path.
- CONTRIBUTING.md: issue tracker URL points at marketplace issues with
  [llm-security] prefix convention.
- CHANGELOG.md: v7.3.1 entry rewritten to reflect actual change
  (URLs corrected to marketplace, not "fixed from one wrong URL to
  another wrong URL").

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/llm-security/package.json

@@ -15,7 +15,7 @@
     "SECURITY.md",
     "CHANGELOG.md"
   ],
-  "homepage": "https://git.fromaitochitta.com/open/claude-code-llm-security",
+  "homepage": "https://git.fromaitochitta.com/open/ktg-plugin-marketplace/src/branch/main/plugins/llm-security",
   "engines": {
     "node": ">=18"
   },
@@ -34,9 +34,10 @@
   "license": "MIT",
   "repository": {
     "type": "git",
-    "url": "https://git.fromaitochitta.com/open/claude-code-llm-security"
+    "url": "https://git.fromaitochitta.com/open/ktg-plugin-marketplace",
+    "directory": "plugins/llm-security"
   },
   "bugs": {
-    "url": "https://git.fromaitochitta.com/open/claude-code-llm-security/issues"
+    "url": "https://git.fromaitochitta.com/open/ktg-plugin-marketplace/issues"
   }
 }