Add skill definitions (SKILL.md + run.py) for all agent tools

skills/fetch_url/SKILL.md

@@ -0,0 +1,9 @@
+---
+name: fetch_url
+description: Fetch a URL and return its text content. HTML is stripped to plain text. Use this to read web pages, documentation, articles, etc.
+parameters:
+  url:
+    type: string
+    description: The URL to fetch
+    required: true
+---