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>

.forgejo/workflows/ci.yaml

@@ -1,6 +1,6 @@
 # CI: every push runs the checks of the pre-push hook (tool/check.sh); a push to main is
 # then deployed to learn.halkyon.eu (deploy/redeploy.sh). Both jobs run on the runner on
-# sahaquiel, directly on the host as an unprivileged user (deploy/runner/). A commit
+# learn.halkyon.eu's server, directly on the host as an unprivileged user (deploy/runner/). A commit
 # that passed the checks before isn't checked again (see "Checked before?").
 #
 # Pushes only, no pull requests: a pull request from a fork would run its own code on
@@ -14,7 +14,7 @@ on:
 
 jobs:
   check:
-    runs-on: sahaquiel
+    runs-on: instance
     timeout-minutes: 45
     env:
       # The Flutter the app is developed with; move it together with the local one.
@@ -143,10 +143,10 @@ jobs:
   deploy:
     needs: check
     if: github.ref == 'refs/heads/main'
-    runs-on: sahaquiel
+    runs-on: instance
     timeout-minutes: 30
     steps:
-      # The key can run nothing but deploy/redeploy.sh, and only from sahaquiel itself.
+      # The key can run nothing but deploy/redeploy.sh, and only from that server itself.
       - name: Deploy to learn.halkyon.eu
         run: >-
           ssh -i "$HOME/.ssh/deploy_trigger" -o IdentitiesOnly=yes -o BatchMode=yes