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/integrations/forge.md

@@ -31,12 +31,14 @@ by a test). swarm-controller mints every agent token with it:
 | `read:notification`  | Poll `GET /notifications` for unread events.                                                            |
 | `write:notification` | Mark notifications read via `PATCH /notifications/threads/{id}`.                                        |
 
-**`CORE_TOKEN_SCOPES`** (`hive-c0re::forge`, for its own `core` user):
-everything in `AGENT_TOKEN_SCOPES` plus `read:admin` and `write:admin`.
-Site-admin membership alone isn't sufficient — Forgejo's token scope gate runs
-before the user-permission check, so `/api/v1/admin/*` returns
-`403 Forbidden` for any token without the admin scope bits, even when
-the bearer is a site admin.
+**`CORE_TOKEN_SCOPES`** (`hive-c0re::forge`, for its own `core` user): the
+same set as `AGENT_TOKEN_SCOPES`, with no admin scope. Forgejo's token scope
+gate runs before the user-permission check, so this token gets
+`403 Forbidden` on `/api/v1/admin/*` even though `core` is a site admin.
+hive-c0re only mints the token when the file at
+`/var/lib/hyperhive/forge-core-token` is missing or the forge rejects it, so
+a token minted with admin scopes keeps them until an operator deletes that
+file and restarts hive-c0re.
 
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