Add catalog update agent

update_catalog/skills/update_docs_file/SKILL.md

@@ -0,0 +1,29 @@
+---
+name: update_docs_file
+description: Insert upgrade content into BUILD-README.md for a target chart version.
+user-invokable: false
+---
+
+# update_docs_file (Skill)
+
+Update a chart version directory's BUILD-README.md by inserting upgrade content.
+
+## Input schema
+```json
+{
+  "repo_path": "string",
+  "docs_file": "string (default: BUILD-README.md)",
+  "version": "string",
+  "content": "string",
+  "overwrite": "boolean"
+}
+```
+
+## Output schema
+```json
+{
+  "success": "boolean",
+  "file_path": "string",
+  "already_existed": "boolean"
+}
+```