fix(amplitude): use git-subdir source to point at plugins/amplitude (#1505)

The amplitude entry used source type "url" which clones the root of
https://github.com/amplitude/mcp-marketplace — a multi-plugin repo
where the actual plugin lives at plugins/amplitude/. Claude Code found
no skills there, so /reload-plugins loaded 0 skills for amplitude.

Switching to "git-subdir" with path "plugins/amplitude" (the same
pattern used by awslabs, bigdata-com, zapier, etc.) makes Claude Code
resolve the correct subdirectory and load all 27 amplitude skills.

Removing the pinned sha so the plugin tracks main, consistent with
how posthog and other unpinned entries behave.

.claude-plugin/marketplace.json

@@ -86,9 +86,10 @@
     {
       "name": "amplitude",
       "source": {
-        "source": "url",
+        "source": "git-subdir",
         "url": "https://github.com/amplitude/mcp-marketplace.git",
-        "sha": "be54ccb66b10593721dd3a31e47b2db20ea02d2f"
+        "path": "plugins/amplitude",
+        "ref": "main"
       },
       "description": "Use Amplitude as an expert analyst — instrument Amplitude, discover product opportunities, analyze charts, create dashboards, manage experiments, and understand users and accounts.",
       "category": "monitoring",