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/allowed-signers

@@ -1,5 +1,5 @@
 # The keys whose signatures deploy/redeploy.sh accepts on main: OpenPGP primary key
 # fingerprints, one per line. The deployed checkout's copy decides, so a new key comes in
 # through a commit signed by a key already listed here, and its public key goes into
-# deploy's GPG keyring on sahaquiel (gpg --import).
+# deploy's GPG keyring on the server (gpg --import).
 B172B9ED76A32EBCABA24A9F07F6FCF405488B64