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

docs/tools/hivectl-cli.md

@@ -600,7 +600,6 @@ Bare `hivectl stop` stops everything; scope flags narrow it to specific sub-agen
 
 * `--agents` — All sub-agent containers
 * `--agent <NAME>` — A specific sub-agent by name. Repeatable: `--agent a --agent b`
-* `--ci` — The CI runner container (`hive-ci`)
 * `--forge` — The forge container (`hive-forge`)
 * `--gateway` — The gateway (`hive-gateway`) — nginx on the host, not a container
 * `--matrix` — The matrix container (`hive-matrix`)
@@ -621,7 +620,6 @@ Bare `hivectl start` restores the agents stopped by the last broad-scope `stop`
 
 * `--agents` — All sub-agent containers
 * `--agent <NAME>` — A specific sub-agent by name. Repeatable: `--agent a --agent b`
-* `--ci` — The CI runner container (`hive-ci`)
 * `--forge` — The forge container (`hive-forge`)
 * `--gateway` — The gateway (`hive-gateway`) — nginx on the host, not a container
 * `--matrix` — The matrix container (`hive-matrix`)
@@ -641,7 +639,6 @@ Bare `hivectl restart` restarts everything; scope flags narrow it.
 
 * `--agents` — All sub-agent containers
 * `--agent <NAME>` — A specific sub-agent by name. Repeatable: `--agent a --agent b`
-* `--ci` — The CI runner container (`hive-ci`)
 * `--forge` — The forge container (`hive-forge`)
 * `--gateway` — The gateway (`hive-gateway`) — nginx on the host, not a container
 * `--matrix` — The matrix container (`hive-matrix`)