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/Caddyfile

@@ -3,7 +3,7 @@
 # production; if it is ever re-enabled, reach it through an SSH tunnel, never through
 # this file.
 #
-# {$DOMAIN} is `learn.halkyon.eu` on sahaquiel, where Caddy obtains and renews a
+# {$DOMAIN} is `learn.halkyon.eu` on its server, where Caddy obtains and renews a
 # Let's Encrypt certificate by itself, and `http://localhost` for the local rehearsal.
 {$DOMAIN} {
 	encode zstd gzip