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/redeploy.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 # Deploys learn.halkyon.eu from main. The only command CI's deploy key may run: deploy's
-# ~/.ssh/authorized_keys on sahaquiel pins that key to this script, from sahaquiel itself:
+# ~/.ssh/authorized_keys on the server pins that key to this script, from the server itself:
 #
 #   restrict,from="127.0.0.1,::1",command="/home/deploy/halkyon-learn/deploy/redeploy.sh" ssh-ed25519 … (CI deploy trigger)
 #
@@ -13,7 +13,7 @@
 # before switching to the new server, and fails unless that server answers afterwards. A failure leaves the checkout at the deployed commit, so
 # running the CI job again retries. By hand, with your own key:
 #
-#   ssh sahaquiel-deploy halkyon-learn/deploy/redeploy.sh <commit>
+#   ssh learn-deploy halkyon-learn/deploy/redeploy.sh <commit>
 set -eu
 
 main() {