v0.91.0 — the DR tier can no longer be applied and dead at the same time (R-39 + R-50b(a))

Closes the agent half of R-39's fleet fix. Requires hub >=0.68.0 for the re-arm signal;
that hub is safe for 0.90.0 agents (unknown key dropped), so it deploys first.

Three compounding defects let a box report `applied` while every PBS request 401'd:

1. The re-key was INVISIBLE. An ep0 re-issue rotates the secret of an existing token, so
   token_id/fingerprint/datastore/namespace come back byte-identical and the descriptor
   content hash never moved — the converged agent short-circuited and never consumed the
   fresh secret. WirePBSDR.SecretGeneration (field-exact with the hub) is what moves the
   hash now, because descriptorHash marshals this struct.

2. The agent could not READ its own credential. It writes /etc/pve/priv/storage/<id>.pw
   through the root wrapper, but that dir is 0700 root:www-data and the wrapper had no
   read verb — so the target resolver got "permission denied" every cycle, warned, and
   skipped. The one loop that could have caught the 401 was blind BY CONSTRUCTION. Adds a
   narrow `read` verb (+ exactly one sudoers line, + a pbsdr-read capability row): one
   secret to stdout, no network, no mutation, never in argv (sudo logs argv), traversal
   refused by the id grammar, the dir allowlist AND a resolved-path prefix assertion.

3. Nothing probed AUTHENTICATION. pbs.ProbeAuth (GET /version + an ErrUnauthorized
   sentinel) runs on the 15-minute collect path and its verdict becomes a loud
   `auth_failed` the hub escalates to a fresh mint. /version needs no datastore, namespace
   or privilege, so a 401 means the CREDENTIAL is bad; 403 is deliberately NOT treated as
   unauthorized, since re-keying a too-narrow token would mint forever without fixing
   anything. A transport error is UNKNOWN, never a rejection — otherwise every network
   blip burns a credential. Recovery self-clears.

R-50b(a): the report now carries the installed wrapper's sha256 so drift against the
vouched manifest value is answerable. Empty = unknown, never drift.

Three red-proofs, all at the assertion level. Removing SecretGeneration fails the re-arm
test with "consume calls=1, want 2". Swallowing the probe result leaves State:applied
AuthFailed:false — the July-18 shape exactly. Notably, deleting the wrapper's id charset
guard alone does NOT open a traversal hole (readlink + the prefix assertion still catch
it), so the isolating red-proof removes BOTH and shows the out-of-tree secret printed —
the layering is real, and a single-guard red-proof would have passed vacuously.

internal/hub/collect.go

@@ -2,8 +2,12 @@ package hub
 
 import (
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"fmt"
+	"io"
 	"log/slog"
+	"os"
 	"time"
 
 	"gitea.dooplex.hu/admin/felhom-agent/internal/capability"
@@ -191,7 +195,8 @@ func (c *Collector) Collect(ctx context.Context) (*HostReport, error) {
 	}
 
 	host := hostMetrics(c.px.Node(), ns)
-	host.CPUTempC = c.cpuTempC(ctx) // slice 9: operator freebie — temp now rides the hub report too
+	host.CPUTempC = c.cpuTempC(ctx)         // slice 9: operator freebie — temp now rides the hub report too
+	host.WrapperSHA256 = pbsWrapperSHA256() // R-50b(a): make privileged-artifact drift answerable
 	report := &HostReport{
 		HostID:       c.hostID,
 		ReportedAt:   c.now().Format(time.RFC3339),
@@ -272,6 +277,27 @@ func (c *Collector) cpuTempC(ctx context.Context) *int {
 	return c.temp.CPUTempC(ctx)
 }
 
+// pbsWrapperPath is the installed PBS-DR apply wrapper. Duplicated from internal/pbsdr.WrapperPath
+// rather than imported, to keep the report collector free of a dependency on the DR bridge.
+const pbsWrapperPath = "/usr/local/sbin/felhom-pbs-apply"
+
+// pbsWrapperSHA256 hashes the installed wrapper for the report (R-50b(a)). Best-effort: a missing or
+// unreadable file yields "", which the hub reads as UNKNOWN rather than as drift — a host that
+// legitimately has no DR wrapper must not light up amber. The file is 0755, so no privilege is
+// needed to read it.
+func pbsWrapperSHA256() string {
+	f, err := os.Open(pbsWrapperPath)
+	if err != nil {
+		return ""
+	}
+	defer f.Close()
+	h := sha256.New()
+	if _, err := io.Copy(h, f); err != nil {
+		return ""
+	}
+	return hex.EncodeToString(h.Sum(nil))
+}
+
 func hostMetrics(node string, ns proxmox.NodeStatus) HostMetrics {
 	h := HostMetrics{
 		Node:             node,