CB-596 round 2: the exec-time argv-prefix fix has no seam — stop and report

herdr protocol 19's agent.start takes a fixed `kind` (herdr resolves the
executable) plus trailing CLI args for that binary; only tab.create/pane.split
accept an env map, and that IS the round-1 pane-creation overlay already
shipped. There is no argv/env control point that runs after the pane's login
shell and before the agent process starts, so the proposed `env NAME=value ...`
argv prefix cannot be implemented against this API. Documented the finding and
corrected bridged.example.yaml's round-1 comments, which had overclaimed that
the overlay survives the login shell.

Kept everything else: added a startup WARN (Bridged.reportMemberCredentialsGap)
when memberCredentials: is absent or its known: list is empty, so CB-592's
protection loss is never silent, mirroring CB-594's reportRequiredSecrets.

bridged/src/main/java/dev/ltms/bridged/Bridged.java

@@ -87,6 +87,11 @@ public final class Bridged {
         // else can fail on a silently-empty one. A daemon started without a login shell (launchd)
         // boots fine either way — this is the only thing that says so out loud.
         reportRequiredSecrets(cfg);
+        // CB-596: an absent (or empty) memberCredentials: block blocks NOTHING — no credential
+        // name is hardcoded any more to fall back on. Say so loudly, the same way a missing
+        // secret is reported above, so upgrading past this commit never silently drops CB-592's
+        // protection.
+        reportMemberCredentialsGap(cfg);
         // CB-559: `cfg` stays the startup snapshot — every validation and every piece of one-time
         // wiring below reads it, and must, because those decisions cannot be unmade. `config` is the
         // live reference the hot paths read per use. Which keys can actually move is ConfigRef's
@@ -615,6 +620,31 @@ public final class Bridged {
         });
     }
 
+    /**
+     * CB-596: {@code known:} empty (block absent entirely, or present but empty) means {@link
+     * BridgedConfig.MemberCredentials#blockedSet()} is empty too — every member pane inherits the
+     * operator's whole secret store, unblocked, exactly the defect this ticket fixes. Unlike a
+     * missing token ({@link #reportRequiredSecrets}), there is no name to point at: the point is
+     * that the block itself is missing. Warn once at startup and say what to add; never refuse to
+     * start over it — see {@link #reportRequiredSecrets} for why a daemon that boots and says
+     * what is wrong beats one that will not boot at all.
+     *
+     * <p>Package-private so the test can capture the log directly, the same way {@link
+     * #requiredSecretEnvVars} is exposed for {@link #reportRequiredSecrets}'s own test.
+     */
+    static void reportMemberCredentialsGap(BridgedConfig cfg) {
+        BridgedConfig.MemberCredentials creds = cfg.memberCredentials();
+        if (creds != null && !creds.known().isEmpty()) {
+            log.info("memberCredentials: {} known name(s), {} allowed — blocking {} on every spawn",
+                    creds.known().size(), creds.allow().size(), creds.blockedSet().size());
+            return;
+        }
+        log.warn("memberCredentials: absent or empty — the daemon will start anyway, and every "
+                + "member pane inherits the operator's WHOLE secret store, unblocked (CB-592's "
+                + "protection is lost). Add a memberCredentials: block (policy/allow/known) to "
+                + "bridged.yaml — see bridged.example.yaml — and restart.");
+    }
+
     /**
      * Poll herdr's {@code ping} until it answers or {@link #HERDR_WAIT_SECONDS} elapses (CB-504).
      *