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.

docs/fleet-management-design.md

@@ -41,13 +41,20 @@ The **server-side control plane + admin SPA is built** and supersedes several
 - **Agent identity confirmed:** the agent reuses the **same TPM identity cert**
   as 802.1X EAP-TLS / NVMe hostnqn — "one identity, three stages". Peer cert
   serial is authoritative; an unenrolled serial is refused.
-- **NOT yet built (recorded intent):** per-host **boot-version assignment**.
+- **On-host agent — telemetry half BUILT (2026-07-05, `files/agent/`).** The
+  gRPC client that feeds this plane. Reuses the host TPM identity for mTLS (read-only
+  `tpmkey.LoadSigner` at `0x81000100` + cert from NV `0x01800100`; private key never
+  leaves the TPM), opens `Session`, sends `Hello` + periodic `HealthSnapshot`
+  (read-only `/proc`+`/sys` probes + `fleet-hostapi` facts). **Telemetry-only:** it
+  receives `DesiredState`/`Command` but only **logs** them. Static out-of-VM CI
+  binary, `2b-agent` provisioner (`enable_agent`), fapolicyd-trusted; mTLS proven
+  against the go-tpm simulator (`files/agent/simtest`).
+- **NOT yet built (recorded intent):** (1) the agent's **reconcile actuators** —
+  apply kiosk_url, reboot_at/REBOOT, drain — deferred until the telemetry half is
+  bench-verified; and (2) per-host **boot-version assignment**.
   `desired.target_version` is stored + pushed, but changing what a host *boots*
-  needs dnsmasq cohort steering + multi-version UKI serving (`docs/TODO.md` #6).
-  Kiosk URL / reboot / drain reconcile via the agent today; the version pin is
-  labelled "recorded intent" in the Lifecycle UI. The **on-host agent** that
-  feeds this plane (extend `fleet-hostapi` with the gRPC report/reconcile client)
-  is likewise still to build.
+  needs dnsmasq cohort steering + multi-version UKI serving (`docs/TODO.md` #6). The
+  version pin is labelled "recorded intent" in the Lifecycle UI.
 
 Everything below is the original design context; treat the as-built note as
 authoritative where they differ.