chore: the servers are named by the domain they serve

Files called the servers by their own host names, which are the
operator's and say nothing a reader needs. They now say which domain a
server serves: learn.halkyon.eu's server (the instance and its CI
runner), the halkyon.app server (Forgejo and the F-Droid repository).

Two names are settings rather than words. CI's runner label becomes
`instance` (`runs-on: instance`, `labels: ["instance:host"]`): the
runner reads its installed copy of deploy/runner/config.yml, so jobs
wait in the queue until that copy is updated and the runner restarted.
The SSH aliases the scripts and docs use become `learn-deploy` and
`fdroid-upload`. AGENTS.md says to keep it so.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

deploy/backup/halkyon-backup.timer

@@ -1,6 +1,6 @@
 # Starts halkyon-backup.service at the start of every hour; the run at 03:00,
-# sahaquiel's time (Europe/Berlin), also prunes and checks, done before the security
-# updates may reboot at 04:00. A run missed while sahaquiel was off happens as soon as
+# the server's time (Europe/Berlin), also prunes and checks, done before the security
+# updates may reboot at 04:00. A run missed while the server was off happens as soon as
 # it is back. Installed as /etc/systemd/system/halkyon-backup.timer.
 [Unit]
 Description=Hourly off-site backup of learn.halkyon.eu