feat(felhomsshd): dedicated OOB sshd instance + port-adaptive belt (H1 Parts 2-4 agent)

internal/felhomsshd: agent-managed felhom-sshd (claim port [8822,2222,8022,62222]
loud-fail-on-exhaustion; render config→sshd -t→reload never-restart-on-change
[SF-2]; operator authorized_keys from the hub block outside ~/.ssh [SF-3]); the
static-table nft belt mutating ONLY @operator_ips + @ssh_port [trap 4]; health/heal
(reset-failed-then-restart with 10min cooldown, NEVER restart onto an invalid
config) + the oob heartbeat stanza. configs/felhom-sshd.service (SAFE, no
RuntimeDirectory [SF-1]). FELHOM_SSHD + FELHOM_OOB sudoers (set-elements only).
oob.enabled config DEFAULT FALSE. Wired into main like wgtunnel.

Non-hollow tests: claim clean/contention/idempotent/exhaustion; config
safe+byte-stable+refuses-:22; belt mutate-then-idempotent + never-touches-rules;
heal no-restart-on-invalid-config + cooldown; status reflects block.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6

configs/felhom-agent.sudoers

@@ -178,4 +178,35 @@ Cmnd_Alias FELHOM_SELFUPDATE = \
   /usr/local/sbin/felhom-selfupdate-guarded commit, \
   /usr/local/sbin/felhom-selfupdate-guarded rollback
 
-felhom-agent ALL=(root) NOPASSWD: FELHOM_MOUNT, FELHOM_DISK, FELHOM_PROVISION, FELHOM_FORMAT, FELHOM_DNSMASQ, FELHOM_GUESTHOOK, FELHOM_INTERMEDIARY, FELHOM_CONTROLLERSWAP, FELHOM_STALELOCK, FELHOM_NETMOUNT, FELHOM_WG, FELHOM_SELFUPDATE
+# Dedicated OOB sshd (TASK H1). The agent manages felhom-sshd like wg-felhom/dnsmasq: it RENDERS the
+# config (Port from its claim) + the operator's authorized_keys, validates with `sshd -t`, and reloads
+# (never restart-on-change [SF-2]). Both install SOURCES are the agent-owned staged files under
+# StateDir; both DESTINATIONS are FIXED. `sshd -t/-T` are the validate/discover reads. The
+# systemctl verbs are SCOPED to felhom-sshd only. reset-failed precedes a deliberate restart [SF-5].
+# NOTHING here can touch the stock sshd, :22, or /etc/ssh.
+Cmnd_Alias FELHOM_SSHD = \
+  /usr/bin/install -o root -g root -m 0644 -- /var/lib/felhom-agent/felhom-sshd/sshd_config /etc/felhom-sshd/sshd_config, \
+  /usr/bin/install -o root -g root -m 0644 -- /var/lib/felhom-agent/felhom-sshd/authorized_keys.felhom-op /etc/felhom-sshd/authorized_keys/felhom-op, \
+  /usr/sbin/sshd -t -f /var/lib/felhom-agent/felhom-sshd/sshd_config, \
+  /usr/sbin/sshd -t -f /etc/felhom-sshd/sshd_config, \
+  /usr/sbin/sshd -T -f /etc/felhom-sshd/sshd_config, \
+  /usr/bin/systemctl enable --now felhom-sshd, \
+  /usr/bin/systemctl reload felhom-sshd, \
+  /usr/bin/systemctl restart felhom-sshd, \
+  /usr/bin/systemctl reset-failed felhom-sshd, \
+  /usr/bin/wg show wg-felhom latest-handshakes
+
+# OOB nft belt (TASK H1). The STATIC table `inet felhom_oob` is installed once by host-install; the
+# agent mutates ONLY its two SETS — @operator_ips (the operator /32) + @ssh_port (the claimed port).
+# SET ELEMENTS ONLY [trap 4]: NO `nft add rule`, NO `nft -f`, NO `flush ruleset/table` — a rule grant
+# would let the agent firewall anything. The agent fine-validates every element (netip / int range)
+# before exec; the trailing wildcards are the coarse allowlist (values only).
+Cmnd_Alias FELHOM_OOB = \
+  /usr/sbin/nft list set inet felhom_oob operator_ips, \
+  /usr/sbin/nft list set inet felhom_oob ssh_port, \
+  /usr/sbin/nft flush set inet felhom_oob operator_ips, \
+  /usr/sbin/nft flush set inet felhom_oob ssh_port, \
+  /usr/sbin/nft add element inet felhom_oob operator_ips *, \
+  /usr/sbin/nft add element inet felhom_oob ssh_port *
+
+felhom-agent ALL=(root) NOPASSWD: FELHOM_MOUNT, FELHOM_DISK, FELHOM_PROVISION, FELHOM_FORMAT, FELHOM_DNSMASQ, FELHOM_GUESTHOOK, FELHOM_INTERMEDIARY, FELHOM_CONTROLLERSWAP, FELHOM_STALELOCK, FELHOM_NETMOUNT, FELHOM_WG, FELHOM_SELFUPDATE, FELHOM_SSHD, FELHOM_OOB

configs/felhom-sshd.service

@@ -0,0 +1,28 @@
+# felhom-sshd.service (TASK H1) — install as /etc/systemd/system/felhom-sshd.service.
+#
+# The dedicated OOB sshd instance: a SECOND sshd on a claimed non-22 port, COEXISTING with the
+# customer's/stock sshd on :22 (never touched). Config is AGENT-RENDERED at /etc/felhom-sshd/sshd_config
+# (Port from the agent's claim); the agent reloads on change (never restart-on-change [SF-2]).
+#
+# CRITICAL [SF-1]: this unit MUST NOT declare `RuntimeDirectory=` — that directive (value `sshd`)
+# removed the SHARED /run/sshd and broke the stock sshd live (SPIKE-felhom-sshd §8). /run/sshd is
+# guaranteed by G1 (tmpfiles + watchdog); the ExecStartPre mkdir is a harmless extra guarantee. The
+# host-install RuntimeDirectory guard will REJECT this unit if the directive is ever added.
+[Unit]
+Description=Felhom OOB sshd (dedicated instance, agent-managed; H1)
+After=network.target
+
+[Service]
+Type=notify
+ExecStartPre=-/usr/bin/mkdir -p /run/sshd
+ExecStartPre=/usr/sbin/sshd -t -f /etc/felhom-sshd/sshd_config
+ExecStart=/usr/sbin/sshd -D -f /etc/felhom-sshd/sshd_config
+# Config-change path: validate then HUP — a bad reload is REFUSED and the running daemon SURVIVES
+# [SF-2]. The agent always writes→sshd -t→reload; it never `restart`s on a config change.
+ExecReload=/usr/sbin/sshd -t -f /etc/felhom-sshd/sshd_config
+ExecReload=/bin/kill -HUP $MAINPID
+KillMode=process
+Restart=on-failure
+
+[Install]
+WantedBy=multi-user.target