swarm-ci-runner: run the forge CI runner as a swarm service

The CI runner was a hive deploy module: hive-ci.nix ran it beside the
forge, and hive-c0re registered it through the forge's admin API with
the `core` token, writing the registration token through hive-priv. It
could only run on the forge's host, and it kept an admin-scoped forge
token on a hive.

Now the runner is a swarm service:

- swarm-ci-runner.nix runs it in a `swarm-ci-runner` container on the
  host that sets deploy.ci-runner.enable, reaching the forge by name.
  It needs neither the forge nor a hive there.
- swarm-controller reads the forge's runner registration token with its
  own admin token and writes it to swarm/services/ci-runner/
  registration-token whenever it differs from what it last wrote. Its
  grant on that path is create/update only.
- The runner's own leaf (CN swarm-ci-runner, role and policy of the same
  name) reads that one path. swarm-bao-ci-runner.service fetches it into
  the runner's EnvironmentFile and restarts the runner on a change.
- A pre-start step removes .runner when the daemon's journal since its
  last start holds Forgejo's "unregistered runner" refusal (the daemon
  exits 1 when its startup Declare gets it), or when .runner names
  another forge address; upstream's pre-start step then registers again.

Removed from the hive side: hive-ci.nix and deploy.forgejo.ci.*,
forge/ci_runner.rs, PrivRequest::RegisterCiRunner and its handler and
ReadWritePaths entry, InfraContainer::Ci with hivectl's --ci and the
dashboard's hive-ci journal entry, HYPERHIVE_FORGE_CI_ENABLED, and
read:admin,write:admin from CORE_TOKEN_SCOPES: nothing else in hive-c0re
calls /api/v1/admin/*. The old ci options, under both swarm.forge.ci and
deploy.forgejo.ci, now fail evaluation naming deploy.ci-runner.

The forge's half of the old ci.enable, the actions/checkout mirror and
DEFAULT_ACTIONS_URL = self, is deploy.forgejo.actionsFromSelf.

Closes #4954

.forgejo/workflows/coverage.yml

@@ -4,7 +4,7 @@ name: coverage
 # than printed into the run log, so a scheduled run leaves something to fetch.
 #
 # ⚠️ The hour is deliberate: at the default job capacity of 1
-# (`services.hyperhive.deploy.forgejo.ci.concurrency`) this job holds the
+# (`services.hyperhive.deploy.ci-runner.concurrency`) this job holds the
 # runner for its whole timeout and everything else queues behind it.
 #
 # Separate from ci.yml because a `workflow_dispatch` trigger there fires every

.forgejo/workflows/flake-update.yml

@@ -11,7 +11,7 @@ name: flake update
 # apply. Stale PRs just stack; newest lock wins.
 #
 # `PR_BOT_TOKEN` (repo secret) is the `flake-bot` account's write token.
-# Self-signed forge cert: not worked around with `curl -k` - hive-ci.nix
+# Self-signed forge cert: not worked around with `curl -k` - swarm-ci-runner.nix
 # already sets SSL_CERT_FILE to a bundle with the hive CA (wiring
 # confirmed; anchor coverage for the swarm forge specifically isn't).
 on: