fleet-agent: on-host TPM-mTLS telemetry client (v1)

Build the on-host agent that pairs with fleet-hostapi and feeds the
fleet-manager control plane (TODO #14 "STILL TO BUILD (a)"). Nothing on the
workstation talked to the manager before this; Monitor + InfluxDB stayed empty.

Scope: telemetry only. The agent reuses the host TPM identity for mTLS (read-only
tpmkey.LoadSigner at 0x81000100 + cert from NV 0x01800100; the private key never
leaves the TPM), opens the long-lived FleetAgent.Session bidi stream, sends Hello
+ periodic HealthSnapshot (load/cpu/temp/mem/tmpfs/nvme/auth/selinux/chrony/kiosk,
probed read-only from /proc+/sys plus hostapi facts), and LOGS but does not act on
the DesiredState/Command it receives. Reconcile actuators (kiosk URL, reboot,
drain) are deferred until the telemetry half is bench-verified.

- files/agent/: new module (go-tpm + grpc), static CGO_ENABLED=0 like enrol/hostapi.
  Copies the enrol tpmkey (+ read-only LoadSigner) and the generated fleet.agent.v1
  proto so it stays self-contained without importing the initramfs enrol module.
- files/agent/simtest/: proves the TPM-signed cert completes a real mTLS client-auth
  handshake (RequireAndVerifyClientCert) against the go-tpm software simulator.
- files/etc/fleet-agent/agent.env: fleet-generic config (manager DNS name, fleet CA
  reused from radius-ca.pem, TPM handle/nvindex, interval). No per-host state.
- fleet-agent.service: hardened unit (tss group for /dev/tpmrm0, INET egress).
- CI builds fleet-agent out-of-VM + runs vet/test + the simulator handshake test.
- Packer: enable_agent flag (default on) + 2b-agent provisioner + fapolicyd trust.
  Post-switch-root only, so no dracut/initramfs changes.
- docs: TODO #14 + fleet-management-design as-built updated.

Requires the host already enrolled (broker approve births the inventory row) —
else the manager returns PermissionDenied, the correct "not enrolled yet" signal.

files/agent/fleet-agent.service

@@ -0,0 +1,53 @@
+[Unit]
+Description=Fleet on-host control-plane agent (telemetry: TPM-mTLS to fleet-manager)
+Documentation=https://git.bitasylum.org/audiobuzz/diskless-workstation/docs/fleet-management-design.md
+# Needs the network up to dial the manager; hostapi is a SOFT dep (facts source) —
+# After, not Requires, so the agent still streams health if hostapi is down.
+After=network-online.target fleet-hostapi.service
+Wants=network-online.target
+
+[Service]
+Type=exec
+EnvironmentFile=/etc/fleet-agent/agent.env
+ExecStart=/usr/local/bin/fleet-agent
+# Always retry — a diskless box has nothing better to do than reconnect. The agent
+# also self-reconnects the stream; Restart covers a hard crash / TPM transient.
+Restart=always
+RestartSec=5
+
+# --- sandboxing (STIG-friendly; looser than hostapi — needs the TPM + egress) ---
+# Runs as a transient unprivileged user in the tss group for /dev/tpmrm0 (the
+# private key stays in the TPM; the agent only signs the TLS handshake through it).
+DynamicUser=yes
+SupplementaryGroups=tss
+# Expose ONLY the TPM resource-manager node (no PrivateDevices — that would hide it).
+DeviceAllow=/dev/tpmrm0 rw
+DevicePolicy=closed
+CapabilityBoundingSet=
+AmbientCapabilities=
+NoNewPrivileges=yes
+ProtectSystem=strict
+ProtectHome=yes
+PrivateTmp=yes
+ProtectKernelTunables=yes
+ProtectKernelModules=yes
+ProtectKernelLogs=yes
+ProtectControlGroups=yes
+ProtectClock=yes
+ProtectHostname=yes
+ProtectProc=default
+RestrictNamespaces=yes
+RestrictRealtime=yes
+RestrictSUIDSGID=yes
+LockPersonality=yes
+MemoryDenyWriteExecute=yes
+SystemCallArchitectures=native
+SystemCallFilter=@system-service
+SystemCallFilter=~@privileged @resources
+# INET egress to the manager (not loopback-only, unlike hostapi); AF_UNIX for the
+# journal, AF_NETLINK for Go's source-address/interface selection.
+RestrictAddressFamilies=AF_INET AF_INET6 AF_UNIX AF_NETLINK
+UMask=0077
+
+[Install]
+WantedBy=multi-user.target