fix: runner DNS resolution and stale registration (#7)

## Summary
- Fix `etc_hosts` templating — dict key was rendered as literal `{{ server_fqdn.value }}` instead of the actual domain
- Clear runner data volume on each deploy to prevent stale registration errors when the token changes
- Use instance-level registration token for global runner access

## Test plan
- [x] `/etc/hosts` in runner container shows `git.maximhutz.com` mapped to `10.0.1.2`
- [x] Runner registers and connects successfully
- [ ] Verify runner picks up jobs from any repo

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #7
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

playbooks/runner.yml

@@ -96,8 +96,14 @@
     - ../vault.yml
     - ../dist/terraform_outputs.yml
   vars:
-    gitea_internal_url: "https://{{ server_fqdn.value }}"
+    gitea_hostname: "{{ server_fqdn.value }}"
+    gitea_internal_url: "https://{{ gitea_hostname }}"
   tasks:
+    - name: Remove stale runner data.
+      community.docker.docker_volume:
+        name: runner-data
+        state: absent
+
     - name: Create runner data volume.
       community.docker.docker_volume:
         name: runner-data
@@ -118,8 +124,7 @@
         state: started
         recreate: true
         restart_policy: unless-stopped
-        etc_hosts:
-          "{{ server_fqdn.value }}": "10.0.1.2"
+        etc_hosts: "{{ {gitea_hostname: '10.0.1.2'} }}"
         volumes:
           - runner-data:/data
           - /var/run/docker.sock:/var/run/docker.sock