docs(ci): add a For operators section

ci.md mixes a genuinely short operator bootstrap step with deep
auto-registration/container-design internals and a full security
writeup an operator does need to read once, but not first.

Adds a short synthesis at the top: enabling is one nix option, the
unsandboxed-build trust tradeoff (fine for a trusted team, gate it if
you take fork PRs), and the disk-maintenance recipe (CI shares the
host nix store with no auto-GC of its own). Every claim checked
against the existing detail further down in this same file.

Part of hyperhive#1898.

docs/ci.md

@@ -3,6 +3,32 @@
 The `hive-ci` module runs a Forgejo Actions runner in a `hive-ci` nixos-container,
 executing CI jobs from `.forgejo/workflows/ci.yml` on every PR.
 
+## For operators
+
+**Enabling it is one line**: `services.hyperhive.forge.ci.enable = true`
+in the host NixOS config. No manual token provisioning — hive-c0re
+registers the runner with the forge automatically.
+
+Two things worth knowing before you rely on it:
+
+- **Only run CI for contributors you trust.** Builds run **unsandboxed**
+  (an nspawn limitation, not a choice) — a malicious build script in a
+  PR could make network requests or write to the container filesystem
+  during the build. Fine for a small trusted-team hive where everyone
+  already has forge access; if you take outside/fork contributions,
+  gate CI behind Forgejo's fork-PR-approval setting or restrict the
+  workflow to push-only triggers (forks can't push to your branches).
+  See *Security* below for the full threat model.
+- **Watch your disk.** CI builds through the host's nix store with no
+  automatic garbage collection of its own — a busy CI day can fill
+  `/nix/store` until every job fails with `ENOSPC`. Add the daily +
+  disk-pressure GC config from *Host store maintenance* below to your
+  host's NixOS config (not optional if you plan to actually use this).
+
+Everything below this point is implementation detail: exact
+auto-registration internals, container design, and the full security
+writeup.
+
 ## CI checks
 
 Three jobs run on every PR (and on `workflow_dispatch` for manual re-triggers):