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

plugins/caveman/plugin.json

@@ -0,0 +1,13 @@
+{
+  "name": "caveman",
+  "description": "Ultra-compressed communication mode from JuliusBrussee/caveman — cuts output tokens while keeping full technical accuracy. Installed straight from the upstream repo, not vendored here.",
+  "version": "1.0.0",
+  "author": "Rootiest",
+  "targets": ["claude-code"],
+  "external": {
+    "claude-code": {
+      "source": "github",
+      "repo": "JuliusBrussee/caveman"
+    }
+  }
+}

plugins/ponytail/plugin.json

@@ -0,0 +1,13 @@
+{
+  "name": "ponytail",
+  "description": "Lazy senior dev mode from DietrichGebert/ponytail — forces the simplest, shortest solution that actually works (YAGNI, stdlib first, no unrequested abstractions). Installed straight from the upstream repo, not vendored here.",
+  "version": "1.0.0",
+  "author": "Rootiest",
+  "targets": ["claude-code"],
+  "external": {
+    "claude-code": {
+      "source": "github",
+      "repo": "DietrichGebert/ponytail"
+    }
+  }
+}