update(plugin-json): point to the correct Semgrep plugin directory

The Semgrep plugin currently does not work correctly when used through
Claude because it is located within a subdirectory of the Semgrep
Marketplace repository. This issue was reported in:
https://github.com/anthropics/claude-plugins-official/issues/450

Previously, this could not be fixed due to a limitation in Claude Code's
handling of plugins located in subdirectories. Support for this was added
with the git-subdir feature, released in v2.1.69:
https://github.com/anthropics/claude-code/issues/30593

A fix for the Semgrep plugin was proposed once this version became the
latest release. Now that v2.1.69+ is available as latest, this PR
implements that fix.

https://claude.ai/code/cse_01RtW9KS12VZNFfWmWY6z9Pu

.claude-plugin/marketplace.json

@@ -704,8 +704,9 @@
       "description": "Semgrep catches security vulnerabilities in real-time and guides Claude to write secure code from the start.",
       "category": "security",
       "source": {
-        "source": "url",
+        "source": "git-subdir",
-        "url": "https://github.com/semgrep/mcp-marketplace.git"
+        "url": "https://github.com/semgrep/mcp-marketplace.git",
+        "path": "plugin"
       },
       "homepage": "https://github.com/semgrep/mcp-marketplace.git"
     },