feat(doorman): badge listener + decoder TDD + deploy scaffolding [#355]

Modern successor of the 2018 Perl doorman (snapshot in legacy/):
bash+coreutils HID decoder, HA webhook dispatch, systemd unit.
Offline decoder suite green (9 tests); shellcheck zero-warning.

Umbrella #345. Work item: https://projects.knownelement.com/issues/355

AGENTS.md

@@ -0,0 +1,51 @@
+# Agent Guidelines — KNEL/doorman
+
+Physical access control: badge-reader listener + tooling. Modern successor
+of the 2018 Perl doorman (`legacy/doorman.pl`, snapshot from
+[KNEL/LegacyTechops](https://git.knownelement.com/KNEL/LegacyTechops)).
+
+House rules layering: `~/.zcode/AGENTS.md` (global) < PFVCluster
+`AGENTS.md` (workspace) < THIS FILE. The more specific file wins.
+
+## Authority (NON-NEGOTIABLE)
+
+- No work without an approved Redmine ticket. Active work: **#355**
+  (code & testing, laptop/office), under umbrella **#345**; deployment is
+  **#356**. NEVER close tickets without explicit human permission.
+- Ticket gate: `echo '#NNN' > .crush/active-ticket` before any modifying
+  work; clear when done.
+- Propose, never implement out-of-scope findings — open a sub-ticket.
+- Security/access changes (door hardware, unlock policy, whitelists) are
+  POLICY decisions — the human decides; the agent implements.
+
+## Access channels
+
+- SSH only (sshd is the only approved remote path). The qemu guest-agent
+  is NEVER an execution channel. `qm guest exec` is banned mechanically.
+- All SSH to fleet hosts routes through the PFVCluster chokepoints:
+  [tests/remote.sh](https://git.knownelement.com/KNEL/PFVCluster/src/branch/main/tests/remote.sh).
+- DNS names only — never IP literals.
+
+## Scope boundary: Home Assistant
+
+The HA side (webhook receiver, whitelist, automations, alerts) is owned by
+the parallel pfv-bms session/repo. This repo ships ONLY the reader-host
+side + the webhook contract documented in README.md. Do not edit
+KNEL/pfv-bms from here.
+
+## Engineering
+
+- TDD red/green for all code (`scripts/test.sh` is the local gate).
+- shellcheck zero-warning (info included) via `bash tests/shellcheck.sh`.
+- This daemon stays bash + coreutils ONLY — the reader Pi gets no package
+  installs, no CPAN, no Python (ruling 2026-08-31).
+- NEVER restart/kill both readers simultaneously: deploy to the dev reader
+  host (ultix-field), verify, then prod (pfvsvrpi) — serial, health-gated.
+- Every command touching a remote host gets `timeout` (30s/120s/300s).
+- Commit+push as work lands; conventional format; `[#NNN]` in subjects.
+
+## Docs
+
+Discourse is the documentation SoR
+([VP TechOps](https://community.turnsys.com/c/vp-techops)). Repo .md files
+are pointers or operational docs that must live beside code (this README).