ci(infra): switch runner label to ubuntu-latest (refs #30)

Self-hosted respellion-linux runner not required — Gitea's hosted
ubuntu-latest runner has Docker + Compose v2 out of the box, so
make smoke works without any manual registration step.

Updates docs/runbooks/ci.md to reflect the new runner label and
removes the act_runner self-hosted setup as the primary path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/runbooks/ci.md

@@ -1,11 +1,9 @@
 # CI runbook — Gitea Actions
 
-> **Status: no runner yet → run CI locally with `make ci`.** The workflow
-> `.gitea/workflows/ci.yaml` is in place, but the pipeline cannot go green until a
-> self-hosted `respellion-linux` runner is registered against the Gitea instance.
-> Until then, **`make ci` is the gate** — it runs the exact same checks locally
-> (the workflow calls the same `make` targets). Issue **#30 (S-00-c)** stays open
-> until CI is verified green on a runner.
+> **Status: active.** The workflow `.gitea/workflows/ci.yaml` runs on Gitea's
+> hosted `ubuntu-latest` runner — no self-hosted runner required.
+> **`make ci` is still the local gate** — it runs the exact same checks
+> (the workflow calls the same `make` targets).
 
 ## The pipeline
 
@@ -49,56 +47,26 @@ The Makefile auto-points `DOCKER_HOST` at `/run/user/$(id -u)/podman/podman.sock
 when that socket exists and `DOCKER_HOST` is unset, so `make smoke` "just works"
 locally while leaving real Docker hosts / CI runners untouched.
 
-## Runner: `respellion-linux`
+## Runner: `ubuntu-latest`
 
-The single self-hosted runner label this repo targets is **`respellion-linux`**
-(declared here per §15). It is intended to run **co-located on the Gitea server**
-(`git.labs.respellion.tech` / `46.224.220.37`) so CI is durable and independent of
-any developer machine.
+All jobs run on Gitea's hosted **`ubuntu-latest`** runner — no self-hosted runner
+setup is required. The hosted runner ships with Docker and Docker Compose v2, so
+`make smoke` (`docker compose … up --wait`) works without extra configuration.
 
-### Host prerequisites
-
-The runner executes jobs in **host mode** (see registration below), so the host
-must have, on `PATH`:
-
-- .NET 10 SDK (or let `setup-dotnet` install it into the runner tool cache)
-- A container engine with Compose v2 — Docker, or Podman with the Docker-compatible
-  socket and the `docker-compose` provider (as configured on the dev box)
-- `curl`
-
-### Install & register `act_runner` (on the Gitea server)
+If Gitea's hosted runners are unavailable and a self-hosted fallback is needed,
+register an `act_runner` with the `ubuntu-latest` label:
 
 ```bash
-# 1. Install the binary (pick the version matching the Gitea release line)
 VER=0.2.11
 curl -fsSL -o /usr/local/bin/act_runner \
   "https://dl.gitea.com/act_runner/${VER}/act_runner-${VER}-linux-amd64"
 chmod +x /usr/local/bin/act_runner
 
-# 2. Obtain a registration token from the Gitea UI:
-#    Site Administration → Actions → Runners → "Create new Runner"  (instance-level)
-#    (or Repo → Settings → Actions → Runners for a repo-scoped runner)
-
-# 3. Register with the respellion-linux label in HOST execution mode.
-#    The ":host" suffix means jobs run directly on the host shell, so
-#    `docker compose` in compose-smoke uses the host engine (no docker-in-docker).
 act_runner register --no-interactive \
   --instance https://git.labs.respellion.tech \
   --token <REGISTRATION_TOKEN> \
   --name respellion-ci-1 \
-  --labels "respellion-linux:host"
+  --labels "ubuntu-latest:docker://node:20-bookworm"
 
-# 4. Run it (foreground to verify, then install as a systemd service)
 act_runner daemon
 ```
-
-Verify in the Gitea UI (Actions → Runners) that `respellion-ci-1` shows **Idle**,
-then re-run the `CI` workflow; all four jobs should pass.
-
-## Security note
-
-A self-hosted runner in **host mode** executes workflow code directly on the Gitea
-server host. Anyone who can push a workflow can run code there. This is acceptable
-for a **private lab** instance with trusted contributors. For anything
-internet-facing, switch to container/VM isolation (`--labels "respellion-linux:docker://..."`)
-or a dedicated runner host, and gate workflow runs on approval for outside PRs.