feat(grpc): PSK-authenticate the site gRPC control plane; drop the vestigial management receptionist registration

Phase 0 of the ClusterClient→gRPC migration
(docs/plans/2026-07-22-clusterclient-to-grpc-plan.md). Standalone hardening: it
closes a gap that exists today and is a precondition for moving command/control
onto gRPC in later phases.

T0.1 — delete the ManagementActor ClusterClientReceptionist registration.
It was built for an out-of-cluster CLI that was never written: the shipped CLI
speaks HTTP Basic to /management, which asks the actor in-process through
ManagementActorHolder. Nothing in the repo ever sent to /user/management. The
actor still runs there; only the cross-boundary advertisement is gone. Six
documents claimed the CLI used ClusterClient — including the CLI's own README
"Architecture Notes" — and are corrected here rather than left to rot.

T0.2 — record, do not port, the dead integration-routing path.
IntegrationCallRequest is unwired at BOTH ends: RouteIntegrationCallAsync has
zero callers anywhere, and RegisterLocalHandler(Integration, …) appears only in
a test, so production always answers "Integration handler not available". It is
excluded from the gRPC contract (28 of 29 commands migrate) rather than
enshrined on an additive-only wire format, and deleting it during a
transport migration would mix a behavioural change into a change whose whole
value is that behaviour is identical. See
docs/known-issues/2026-07-22-integration-call-routing-is-dead-code.md.

T0.3 — preshared-key authentication on SiteStreamService.
The service shipped with no auth at all: plaintext h2c, no interceptor, so
anything that could reach a site node's :8083 could open a live data stream or
read audit rows back via PullAuditEvents/PullSiteCalls. ControlPlaneAuthInterceptor
now gates /sitestream.SiteStreamService/ — modeled on LocalDbSyncAuthInterceptor
(constant-time compare, fail-closed, PermissionDenied) but gating a SET of
service prefixes so phases 1A/1B add services rather than interceptors. LocalDb
sync keeps its own separate key: it authenticates the pair partner, not central,
and collapsing the two would make a site's central-facing key also admit writes
into its database.

Keys are per site (SB-GRPC-PSK-<siteId>), never fleet-wide, so a compromised
site yields only its own. Central attaches them through ControlPlaneCredentials,
which binds CallCredentials to the channel — covering unary and streaming
uniformly, and letting the key resolve asynchronously, which a client
interceptor could not do without blocking. All three central→site channel
creation sites go through it (SiteStreamGrpcClient and both audit pull invokers);
the pull invokers' channel caches are re-keyed by (site, endpoint) because
credentials are per-site and bound to the channel.

Two decisions beyond the plan:

  * StartupValidator now requires GrpcPsk on Site nodes. The plan specified only
    the runtime gate, but fail-closed with no boot check produces a node that
    joins, answers heartbeats and reports healthy while refusing every stream,
    audit pull and telemetry ingest — silent and total. Same reasoning as the
    existing inbound API-key pepper rule.

  * Added Communication:SitePsks as a central-side key map. The plan assumed
    central would read the store, seeded via a dev KEK; the docker rig
    deliberately boots with no master key, so store-only resolution would leave
    it unable to dial its own sites. The store stays primary — it is the only
    source that can serve a site added at runtime — with the map covering
    key-less hosts and one-off pins. Neither source falling back to
    "unauthenticated" is the invariant.

T0.4 — dev keys on both rigs and tests.
34 tests. The seven that matter most exercise a real in-process gRPC stack over
TestServer: the unit tests on either side of the wire would both stay green if
the halves disagreed, and gRPC refuses call credentials on a plaintext channel
by default — the UnsafeUseInsecureChannelCallCredentials opt-in is only provable
by making a real call. They confirm correct key passes on unary AND streaming,
wrong key and no-credentials both get PermissionDenied, and an unresolvable key
fails the call with nothing reaching the service.

OPERATIONAL: a site node upgraded to this build without a key will not boot.
That includes the gitignored deploy/wonder-app-vd03/ overlay.

docs/requirements/Component-Host.md

@@ -125,7 +125,11 @@ The Host bootstraps the Akka.NET actor system from a **hand-assembled, injection
 
 ### REQ-HOST-6a: ClusterClientReceptionist (Central Only)
 
-On central nodes, the Host must configure the Akka.NET **ClusterClientReceptionist** and register the ManagementActor with it. This allows external processes (e.g., the CLI) to discover and communicate with the ManagementActor via ClusterClient without joining the cluster as full members. The receptionist is started as part of the Akka.NET bootstrap (REQ-HOST-6) on central nodes only.
+On central nodes, the Host must configure the Akka.NET **ClusterClientReceptionist** and register the **CentralCommunicationActor** with it, so that site clusters' ClusterClients can reach the central command/control endpoint without joining the central cluster.
+
+**The ManagementActor is NOT registered with the receptionist** (removed 2026-07-22). That registration was written for an out-of-cluster CLI that was never built: the shipped CLI speaks HTTP Basic to the central `/management` endpoints, which ask the ManagementActor **in-process** via `ManagementActorHolder` (`ManagementEndpoints.cs`). No sender to `/user/management` existed anywhere in the repo, so the registration only widened the cluster-client surface for nothing. The actor itself still runs at `/user/management`; only its cross-boundary advertisement is gone.
+
+> **Migration note.** This receptionist registration — and the `CentralCommunicationActor` one that remains — are scheduled for deletion once the site↔central transport moves to gRPC. See `docs/plans/2026-07-22-clusterclient-to-grpc-plan.md`.
 
 ### REQ-HOST-7: ASP.NET Web Endpoints