feat(infra): add ci-runners.sh wrapper for the runner stack

Mirrors the convenience-script pattern we adopted for
infra/local/dev.sh: typing
`docker compose -f infra/ci-runners.compose.yml ...` for routine
ops (status, restart, log tail) gets old fast, the pre-pull of
the catthehacker job images is documented but easy to forget,
and the "rotation of one runner at a time" tip in
infra/README.md is a sequence the contributor was supposed to
hand-roll every time.

`infra/ci-runners.sh` exposes:

  ./ci-runners.sh up                  # bring up runner-1..3
  ./ci-runners.sh up --prepull        # pre-pull job images first
  ./ci-runners.sh down                # stop + remove (preserves
                                       data/runner-N/.runner creds)
  ./ci-runners.sh restart <runner>    # restart one runner
  ./ci-runners.sh rotate              # rolling restart with a 15 s
                                       pause between each — keeps
                                       at least N-1 runners online
                                       through a config refresh
  ./ci-runners.sh status              # docker compose ps
  ./ci-runners.sh logs [runner]       # follow logs
  ./ci-runners.sh pull-images         # idempotent image refresh

Anything else is passed through to
`docker compose -f ci-runners.compose.yml ...`. The destructive
`down -v` (wipes data/, forces re-registration with a fresh
Gitea token) is intentionally NOT exposed as a verb — invoke
docker compose directly so the path is explicit at the typing
level.

`infra/README.md` updated:

- Inventory table at the top picks up the script.
- "First-time registration" walkthrough swaps the explicit
  docker pull / docker compose up steps for `./ci-runners.sh
  up --prepull`.
- New "Convenience script — ci-runners.sh" subsection with the
  cheat-sheet table.
- "Operational tips" rephrased to point at the script's `rotate`
  / `restart` / `logs` verbs as the canonical commands; the
  raw-docker-compose form is kept in parentheses as the
  underlying mechanism.
- "Adding a fourth runner" tip now reminds to update the
  RUNNERS=() array at the top of the script.

The 15 s pause in `rotate` is a conservative approximation —
act_runner doesn't expose a Compose healthcheck, so we can't
poll for ready. Adjust the constant if reality argues for a
different value.

Smoke-tested help + arg-validation paths on the host.

infra/ci-runners.sh

@@ -0,0 +1,169 @@
+#!/usr/bin/env bash
+# Convenience wrapper around infra/ci-runners.compose.yml. Documented
+# in infra/README.md → "CI runners" → "Convenience script".
+#
+# Hides the compose-file path on every command, automates the
+# pre-pull of the job images that ADR-0015 §"Job image pinning and
+# pre-pull" otherwise leaves to a manual step, and exposes the
+# "one-runner-at-a-time" rotation pattern as a single verb so the CI
+# pipeline is never starved during a config refresh.
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+COMPOSE_FILE="${SCRIPT_DIR}/ci-runners.compose.yml"
+
+# Runner service names defined in the compose file. Update when a
+# fourth runner is added (rare; documented in infra/README.md).
+RUNNERS=(runner-1 runner-2 runner-3)
+
+# Pre-pulled job images. The labels in ci-runners.compose.yml point at
+# these tags; act_runner with `force_pull: false` (per
+# infra/runner-config.yaml) refuses to start a job if the image is not
+# already cached locally.
+JOB_IMAGES=(
+  "catthehacker/ubuntu:act-22.04"
+  "catthehacker/ubuntu:full-22.04"
+)
+
+# Seconds to wait between rotation steps. The runners do not expose a
+# Compose healthcheck; this is a conservative pause to let act_runner
+# finish its registration handshake before we touch the next one.
+ROTATE_WAIT_SECONDS=15
+
+dc() {
+  docker compose -f "$COMPOSE_FILE" "$@"
+}
+
+usage() {
+  cat <<'USAGE'
+Usage: ./infra/ci-runners.sh <command> [args]
+
+Commands:
+  up [--prepull]        Bring up the three runner containers.
+                        --prepull also fetches the act_runner job images
+                        (`catthehacker/ubuntu:act-22.04` and
+                        `:full-22.04`) on the host first — required
+                        because `runner-config.yaml` sets
+                        `container.force_pull: false`.
+
+  down                  Stop and remove the runner containers.
+                        Persistent state under `data/runner-N/.runner`
+                        survives — the runners re-authenticate from
+                        their stored credentials on the next `up`.
+                        For a clean wipe (forces re-registration with
+                        a fresh Gitea token), invoke
+                        `docker compose -f ci-runners.compose.yml down -v`
+                        directly.
+
+  restart <runner>      Restart one runner.
+
+  rotate                Rolling restart of every runner (runner-1,
+                        wait, runner-2, wait, …). The pause between
+                        steps lets the previous runner finish its
+                        registration handshake before the next one
+                        goes down, so the CI pipeline always has at
+                        least N-1 runners online.
+
+  status                docker compose ps for the runner services.
+
+  logs [runner]         Follow logs (one runner or all of them).
+
+  pull-images           Pre-pull / refresh the act_runner job images
+                        on the host. Idempotent (Docker reports
+                        "Image is up to date" when nothing changed).
+
+  help                  Show this help.
+
+  <other> [args...]     Anything else is passed through to
+                        `docker compose -f ci-runners.compose.yml ...`.
+
+Examples:
+  ./infra/ci-runners.sh up --prepull
+  ./infra/ci-runners.sh rotate
+  ./infra/ci-runners.sh logs runner-2
+  ./infra/ci-runners.sh restart runner-1
+USAGE
+}
+
+require_args() {
+  local verb="$1"
+  local n="$2"
+  local hint="$3"
+  shift 3
+  if [[ $# -lt $n ]]; then
+    echo "Error: '$verb' requires $hint." >&2
+    echo "Try: ./infra/ci-runners.sh help" >&2
+    exit 64
+  fi
+}
+
+assert_known_runner() {
+  local name="$1"
+  for r in "${RUNNERS[@]}"; do
+    if [[ "$r" == "$name" ]]; then
+      return 0
+    fi
+  done
+  echo "Error: unknown runner '$name'. Known: ${RUNNERS[*]}." >&2
+  exit 64
+}
+
+pull_images() {
+  echo "Pre-pulling job images on the runner host:"
+  for img in "${JOB_IMAGES[@]}"; do
+    echo "  • $img"
+    docker pull "$img"
+  done
+}
+
+cmd="${1:-help}"
+[[ $# -gt 0 ]] && shift
+
+case "$cmd" in
+  up)
+    if [[ "${1:-}" == "--prepull" ]]; then
+      pull_images
+      shift
+    fi
+    dc up -d "$@"
+    ;;
+  down)
+    dc down "$@"
+    ;;
+  restart)
+    require_args restart 1 "a runner name (e.g. runner-1)" "$@"
+    assert_known_runner "$1"
+    dc restart "$1"
+    ;;
+  rotate)
+    echo "Rolling restart of ${#RUNNERS[@]} runners — ${ROTATE_WAIT_SECONDS}s pause between each."
+    for r in "${RUNNERS[@]}"; do
+      echo
+      echo "→ Restarting $r…"
+      dc restart "$r"
+      if [[ "$r" != "${RUNNERS[-1]}" ]]; then
+        echo "  Waiting ${ROTATE_WAIT_SECONDS}s before the next runner."
+        sleep "$ROTATE_WAIT_SECONDS"
+      fi
+    done
+    echo
+    echo "✓ Rotation complete."
+    ;;
+  status | ps)
+    dc ps "$@"
+    ;;
+  logs)
+    dc logs -f "$@"
+    ;;
+  pull-images)
+    pull_images
+    ;;
+  help | -h | --help | "")
+    usage
+    ;;
+  *)
+    # Pass-through.
+    dc "$cmd" "$@"
+    ;;
+esac