ci: remove GitHub CI — native git.hanzo.ai + cd.hanzo.ai only

This workflow ran on a self-hosted actions-runner-controller pool. ARC is
decommissioned: CI executes inside git.hanzo.ai (Gitea Actions, .hanzo/workflows)
and delivery is cd.hanzo.ai reconciling the reviewed image pin in
hanzoai/universe. GitHub is a mirror and runs nothing.

Removed rather than repointed — there is no GitHub-side pool to move to, and
leaving it would keep ARC load-bearing.

.github/workflows/first-interaction.yml

@@ -1,36 +0,0 @@
-name: First interaction greeting
-
-on:
-  pull_request:
-    types: ["opened"]
-  issues:
-    types: ["opened"]
-
-permissions:
-  pull-requests: write
-  issues: write
-
-jobs:
-  greeting:
-    runs-on: hanzo-build-linux-amd64
-    steps:
-      - uses: actions/first-interaction@v1
-        with:
-          repo-token: ${{ secrets.GITHUB_TOKEN }}
-          pr-message: |
-            Welcome @${{github.actor}} :wave:
-
-            Thank you for creating your first Pull Request! 
-
-            One of our team members will review it and get back to you as soon as possible.
-
-            In the meantime feel free to hop into our community in [Discord](https://discord.gg/rCpqnD6G6p)
-
-            Thanks again, and welcome to the community! :smiley:
-
-          issue-message: |
-            Welcome @${{github.actor}} :wave:
-
-            thank you for opening your first issue! We will get back to you shortly.
-
-            Meanwhile, please feel free to hop into our community in [Discord](https://discord.gg/rCpqnD6G6p)