fix(ops): run heavy commands under a hard memory cap to stop host OOM kills

The host runs with vm.overcommit_memory=0 and no swap, so a process that
grows past free memory makes the kernel OOM-kill across the whole machine
-- the Turbopack build (commit 3d828a61) could take out the database,
nginx or the live release.

Add scripts/with-memory-cap.sh: it moves a command into its own systemd
scope with MemoryMax, so only that cgroup gets OOM-killed (verified: a
Turbopack build died at its 6GB cap, host untouched). Build/analyze/dev/
test*/typecheck now run under explicit caps; ulimit -v is only an explicit
opt-in because it bounds virtual address space per process and 10g/20g both
break V8-based builds. Docker and GitLab builds run in their own isolated
containers with a read-only cgroupfs and opt out explicitly (webpack +
--max-old-space-size stay their bound).

Measured: webpack build peaks ~6.5GB RSS, so 10GB leaves headroom within
the 23.5GB host.

.gitlab-ci.yml

@@ -1,5 +1,12 @@
 image: node:26
 
+# Runner containers have no systemd, so scripts/with-memory-cap.sh cannot
+# enforce an RSS cap there and correctly refuses to run unbounded. These
+# builds are already isolated inside their own runner container (not the
+# host) and use the webpack builder; opt out explicitly on purpose.
+variables:
+  CMS_MEMORY_CAP_BACKEND: "none"
+
 stages:
   - test
   - build