feat: add external-source plugins for third-party caveman/ponytail

Lets a plugin.json declare an `external` target->source map instead of
carrying local content, so the generator emits that Claude Code
marketplace source (e.g. a GitHub repo) verbatim rather than a local
dist path. Adds caveman and ponytail as external entries, installed
straight from their upstream repos with nothing vendored or copied.
Claude Code-only for now — agy has no equivalent external-source
mechanism, and both plugins' hooks/MCP assume Claude Code's runtime.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014YVWWAnvR5TDQzmfY3RXG9

.claude-plugin/marketplace.json

@@ -6,6 +6,13 @@
   },
   "description": "Reusable AI skills for Claude Code and Antigravity CLI",
   "plugins": [
+    {
+      "name": "caveman",
+      "source": {
+        "source": "github",
+        "repo": "JuliusBrussee/caveman"
+      }
+    },
     {
       "name": "core-essentials",
       "source": "./dist/claude-code/core-essentials"
@@ -30,6 +37,13 @@
       "name": "github-mcp",
       "source": "./dist/claude-code/github-mcp"
     },
+    {
+      "name": "ponytail",
+      "source": {
+        "source": "github",
+        "repo": "DietrichGebert/ponytail"
+      }
+    },
     {
       "name": "readme-sync-audit",
       "source": "./dist/claude-code/readme-sync-audit"