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/go.mod

@@ -0,0 +1,28 @@
+// fleet-agent — the on-host steady-state control-plane client (telemetry v1).
+//
+// Runs in the BOOTED system (post switch-root), NOT the initramfs. It reuses the
+// host's TPM identity (signing key at 0x81000100 + cert in NV 0x01800100) as an
+// mTLS client cert, opens the long-lived FleetAgent.Session bidi stream to the
+// fleet-manager, sends Hello + periodic HealthSnapshot, and (v1) logs but does not
+// act on the desired-state/commands it receives. See docs/fleet-management-design.md.
+//
+// Unlike fleet-hostapi (stdlib-only) this needs gRPC + go-tpm; like fleet-enrol it
+// is a static CGO_ENABLED=0 binary whose deps are proxy-fetched, go.sum-PINNED, and
+// built out-of-VM in CI (never enters the reproducible image build's toolchain).
+// Dep versions track serving/fleet-manager/go.mod (the peer of this wire contract).
+module fleet-agent
+
+go 1.26
+
+require (
+	github.com/google/go-tpm v0.9.8
+	google.golang.org/grpc v1.82.0
+	google.golang.org/protobuf v1.36.11
+)
+
+require (
+	golang.org/x/net v0.53.0 // indirect
+	golang.org/x/sys v0.43.0 // indirect
+	golang.org/x/text v0.36.0 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20260414002931-afd174a4e478 // indirect
+)