security: version the CIS Tier-A hardening pass

The script that sets the SSH, PAM, pwquality and auditd baseline on twelve
containers existed only as a root-owned copy on the machines it hardens.
Bring it into the repo so a change reaches one place instead of twelve.

Two substantive changes over the copy that shipped on 2026-07-24:

- Deny forwarding per option instead of via DisableForwarding. Same effect,
  but DisableForwarding overrides every other forwarding option and is
  invisible in sshd -T, which makes a rejected port-forward read as a
  configuration that should work.
- Quote the command substitution in MODDIR (SC2046).

The header and README now record the rollout command, the containers left
unhardened as break-glass foundation, and why host-specific exceptions must
live in a drop-in that sorts after 99-cis-hardening.conf.

README.md

@@ -52,6 +52,24 @@ Shared libs: [`lib/build.func`](lib/build.func) (host-side: prompts, LXC create,
 - Secrets are generated on the target host and live in `0600` files — never in the repo, the wiki or chat logs.
 - **Runner separation:** the nexus runner is repo-scoped and lives on the production LXC *because* it holds deploy rights; the general `runner` LXC is instance-wide *because* it holds none. Don't mix these scopes.
 
+### CIS Tier-A hardening
+
+[`install/cis-tierA.sh`](install/cis-tierA.sh) applies the Tier-A baseline to an existing Debian 12 LXC: SSH drop-in, `login.defs` aging + YESCRYPT, pwquality/faillock, PAM (pwquality, pwhistory, faillock, `nullok` removed) and auditd rules. It is idempotent, backs everything up to `/root/cis-hardening-backup-<ts>/`, gates the SSH restart on `sshd -t` and rolls PAM back if a referenced module is missing.
+
+```bash
+pct push <id> install/cis-tierA.sh /root/cis-tierA.sh
+pct exec <id> -- bash /root/cis-tierA.sh
+```
+
+Applied to authentik first (2026-07-24), then to 11 further containers. The PVE hosts and the Wazuh manager VM stay unhardened on purpose — they are the break-glass foundation.
+
+Two things worth knowing before touching it:
+
+- **Section 1 rewrites `99-cis-hardening.conf` wholesale on every run.** Host-specific exceptions belong in a separate drop-in that sorts *after* it (e.g. `99-zz-local-forward.conf`), never in that file. A `Match` block extends until the next `Match` — across `Include` file boundaries — so such a drop-in has to stay alphabetically last.
+- **Forwarding is denied per option, not via `DisableForwarding`.** Both are equivalent in effect, but `DisableForwarding` overrides every other forwarding option *and* does not appear in `sshd -T` output. A denied port-forward then reports `administratively prohibited` while `sshd -T` cheerfully claims `allowtcpforwarding yes`, which costs hours to diagnose.
+
+Caveat: auditd is a no-op in unprivileged LXCs (the host owns the audit subsystem); the script probes for it and removes the package again if it cannot load rules. Together with the Section-1 partition/kernel checks that are equally N/A in a container, the achievable SCA score stays well below 100 %.
+
 ## Contributing
 
 **All changes go through a pull request with cross-review** (Claude Code ↔ Codex, or a human) — no direct pushes to `main`. This rule exists because of two real incidents: a pasted VLAN tag carrying an invisible non-UTF-8 byte broke `pct create` mid-run, and the "missing `source build.func`" bug shipped twice — caught in review on the nexus-db PR, but reaching production via an un-reviewed authentik commit (see wiki → Lessons).
@@ -65,7 +83,8 @@ Build new app prompts on `prompt_validated`/`require_valid` from `lib/build.func
 ```
 .
 ├── ct/                 # Host-side scripts, one per app
-├── install/            # In-container installers (+ nexus-runtime.sh re-provisioner)
+├── install/            # In-container installers (+ nexus-runtime.sh re-provisioner,
+│                       #   cis-tierA.sh hardening pass)
 ├── lib/
 │   ├── build.func      # Shared host-side helpers (prompts, LXC create, bootstrap)
 │   └── install.func    # Shared in-container helpers (apt, systemd, users, http-wait)