move ci job scipts into an actual script

This will make it possible/easier to:

* share code between jobs
* run jobs locally

bin/job

@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+
+job_artifacts() (
+    set -euo pipefail
+
+    nix-build-and-cache packages
+)
+
+job_ci() (
+    set -euo pipefail
+
+    nix-build-and-cache ci
+    direnv exec . engage
+)
+
+job_pages() (
+    set -euo pipefail
+
+    nix build .#website-root
+    cp --recursive --dereference result public
+)
+
+bail() (
+    set -euo pipefail
+
+    echo
+    echo "Job failed"
+    exit 1
+)
+
+run() (
+    set -euo pipefail
+
+    if [[ -z "${1+x}" ]]; then
+        echo "You must supply a job to run. Available jobs:"
+        declare -F | rg \
+            --only-matching \
+            --color never \
+            --replace '* $1' \
+            '^declare -f job_(.*)$'
+
+        exit 1
+    fi
+
+    job="$1"
+
+    cd "$(git rev-parse --show-toplevel)"
+
+    job_"$job" || bail
+)
+
+run "$@"