chore(plugins): remove stale SKILL.md stubs from plugin skill dirs

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

plugins/gitea/skills/gitea-issues/SKILL.md

@@ -1,10 +0,0 @@
----
-name: gitea-issues
-description: >
-  Use when you want to create, list, update, label, or close Gitea issues
-  via the Gitea MCP server. Stub — full content not yet written.
-metadata:
-  category: operate
----
-
-> **Stub.** This skill is a placeholder. Content will be authored in a future workstream.