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.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 # Backs up learn.halkyon.eu off-site. Runs every hour from
-# deploy/backup/halkyon-backup.timer, as deploy on sahaquiel; deploy/backups.md says
+# deploy/backup/halkyon-backup.timer, as deploy on the server; deploy/backups.md says
 # how it was set up and how to restore.
 #
 # Everything people keep is in the database, their files too (Serverpod's database
@@ -12,7 +12,7 @@
 # started since, that is everything: their app no longer finds its session and starts
 # anew (deploy/backups.md). After 3 months, a deleted account is gone from every
 # backup (the privacy page says so). The box's own automatic snapshots, which only
-# Hetzner's Console can remove, keep the backups safe from a taken-over sahaquiel,
+# Hetzner's Console can remove, keep the backups safe from a taken-over server,
 # which can delete what restic wrote.
 #
 # Failures go to the journal only (decided 2026-10-03):