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/TODO.md

@@ -1396,13 +1396,31 @@ the design forks resolved:
   + agent (`:9600`) listeners to the mgmt net; run `server-pki.sh` when the agent
   lands; decide `:9500` vs a 443 proxy; digest-pin the images (invariant #12). See
   DEPLOY.md "Hardening status".
-- **STILL TO BUILD:** (a) the **on-host agent** — the gRPC report/reconcile client
-  that pairs with `fleet-hostapi` (feeds this plane; nothing on the host talks to
-  the manager yet); (b) **per-host boot-version assignment** — `target_version` is
+- **[~] (a) the on-host agent — TELEMETRY HALF BUILT 2026-07-05 (`files/agent/`).**
+  The gRPC client that pairs with `fleet-hostapi` and feeds this plane. It reuses
+  the host **TPM identity** for mTLS (signing key at `0x81000100` via
+  `tpmkey.LoadSigner` — a read-only copy of the enrol `tpmkey`; cert from NV
+  `0x01800100`), opens the long-lived `Session` stream, sends `Hello` + periodic
+  `HealthSnapshot` (load/cpu/temp/mem/tmpfs/nvme/auth/selinux/chrony/kiosk probed
+  read-only from `/proc`+`/sys` + hostapi facts), and — **telemetry-only v1** —
+  **receives `DesiredState`/`Command` but only LOGS them** (zero host mutation).
+  Static `CGO_ENABLED=0` binary built out-of-VM in CI (like enrol/hostapi), staged
+  by the `2b-agent` provisioner (`enable_agent`, default on), fapolicyd-trusted,
+  fleet-generic config in `files/etc/fleet-agent/agent.env`. mTLS handshake proven
+  end-to-end against the go-tpm software simulator (`files/agent/simtest`). No
+  dracut/initramfs changes (post-switch-root only). **STILL TO BUILD (deferred):**
+  the **reconcile actuators** — apply kiosk_url, reboot_at/REBOOT, drain — wired to
+  be received but not acted on; land after the telemetry half is bench-verified.
+  **BENCH-VERIFY:** on an enrolled booted host, `systemctl status fleet-agent`
+  active; manager logs `agent connected: <serial>`; SPA Monitor shows the host
+  online + health; InfluxDB `fleet_health` accrues points. Requires the host
+  already enrolled (broker approve birthed its inventory row) — else the manager
+  returns `PermissionDenied` (the correct "not enrolled yet" signal).
+- **STILL TO BUILD:** (b) **per-host boot-version assignment** — `target_version` is
   stored + pushed but does not change what a host *boots*; that needs dnsmasq
   cohort steering + multi-version UKI serving (**#6**). Kiosk URL / reboot / drain
-  reconcile via the agent once (a) lands. Version pin is labelled "recorded intent"
-  in the Lifecycle UI.
+  reconcile via the agent's deferred actuator half (above). Version pin is labelled
+  "recorded intent" in the Lifecycle UI.
 
 Depends on the step-ca fleet CA (#8) for the mTLS identity.