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

swarm-controller/src/forge/objects.rs

@@ -84,9 +84,8 @@ const OPERATORS_TEAM_UNITS: [&str; 7] = [
 ];
 
 /// Periodic sync interval for pull-mirrors. Forgejo syncs mirrors on-access by
-/// default, which re-introduces external DNS latency on every `git clone` (the
-/// hive-ci runner shares the host netns and is therefore affected by host
-/// resolver blips). A fixed periodic interval isolates CI from transient DNS
+/// default, which re-introduces external DNS latency on every `git clone`. A
+/// fixed periodic interval isolates CI from transient DNS
 /// failures — a stale mirror is acceptable; a broken clone because of a
 /// momentary DNS blip is not. Written in the form Forgejo echoes back, so an
 /// unchanged mirror compares equal and is not re-patched.
@@ -118,7 +117,7 @@ Do **not** push directly to `main` — agents have read-only access.
 ";
 
 /// The operator-declared pull-mirrors (nix `deploy.forgejo.mirrors`, plus the
-/// CI-auto `actions/checkout` entry), JSON-encoded by `hive-forge/default.nix`.
+/// `actions/checkout` entry `deploy.forgejo.actionsFromSelf` adds), JSON-encoded by `hive-forge/default.nix`.
 /// Absent or empty means no mirrors.
 const MIRRORS_ENV: &str = "SWARM_CONTROLLER_FORGE_MIRRORS";