feat: add plugin-create skill and plugin template

Adds the plugin-create skill (.agents/skills/plugin-create/) that scaffolds
a new plugin from templates/plugin/, substitutes PLUGIN_NAME/AUTHOR_* markers,
registers the plugin in marketplace.json, runs claude plugin validate ., and
hands off to /marketplace-architect. Includes evals and the generic template
with all components (agents, skills, hooks, MCP, bin, docs) with per-directory
READMEs explaining what goes where and which tool reads each directory.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/plugin/hooks/README.md

@@ -0,0 +1,27 @@
+# hooks/
+
+**Claude Code only.** Hook definitions that run in response to Claude Code events.
+
+The `hooks.json` in this directory is read by Claude Code. Structure:
+
+```json
+{
+  "hooks": {
+    "PostToolUse": [
+      {
+        "matcher": "Bash",
+        "hooks": [
+          { "type": "command", "command": "echo 'tool used'" }
+        ]
+      }
+    ]
+  }
+}
+```
+
+Supported events: `PreToolUse`, `PostToolUse`, `Notification`, `Stop`.
+
+Use `${CLAUDE_PLUGIN_ROOT}` to reference scripts inside this plugin — the plugin runs
+from a cache path after install, not its original repo location.
+
+For GitHub Copilot CLI hooks, see `hooks.json` at the plugin root.