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/CLAUDE.md

@@ -225,11 +225,19 @@ Standalone CLI makes zero network calls in default mode. Schrems II compatible i
 
 Scan reports are stored in `reports/` as `.docx` (for sharing) with `.md` source.
 
-## Public Repository
+## Distribution
 
-Published as standalone repo: `https://git.fromaitochitta.com/open/claude-code-llm-security`
+This plugin lives in the `ktg-plugin-marketplace` monorepo at
+`https://git.fromaitochitta.com/open/ktg-plugin-marketplace` under
+`plugins/llm-security/`. It is not published as a standalone repo —
+users install it via the Claude Code marketplace mechanism:
 
-Pushed via `git subtree push --prefix=plugins/llm-security` from the plugin-marketplace monorepo.
+```bash
+claude plugin marketplace add https://git.fromaitochitta.com/open/ktg-plugin-marketplace.git
+```
+
+Issues, bug reports, and security disclosures all route to the
+marketplace repo.
 
 ## State