swarm-bao: grant the controller read on the agent credential prefix

mint_and_verify reads the queue credential back before writing it, so a
re-run keeps the value a live agent already authenticates with. that read
is read_optional, which maps only a 404 to absence — so with create/update
alone every mint aborted on a 403 at its first store read.

read on the same paths the stanza already grants create and update, and
nothing else: no list, no delete, no patch.

nix/host-modules/swarm-bao.nix

@@ -180,9 +180,17 @@ let
   # `secret`, and the engine inserts `data/`. Matching the code's spelling
   # literally would grant nothing.
   #
-  # Write-only on purpose. The controller mints these; nothing in its job reads
-  # one back, and a read capability here would let it recover every agent's
-  # credentials rather than merely replace them.
+  # `read` is there because the controller mints idempotently: `mint_and_verify`
+  # reads the queue credential back before writing so that a re-run keeps the
+  # value a live agent is already authenticating with instead of rotating it.
+  # That read is `read_optional`, which treats only a 404 as absence — a denial
+  # stays an error on purpose, so without `read` the first re-run of a mint
+  # aborts on a 403 rather than concluding the path is empty.
+  #
+  # It is the whole of the widening: `read` on the paths this same stanza
+  # already lets the controller create and replace, and no `list`, so it can
+  # only fetch a credential for an agent it was told the name of — which is an
+  # agent it could already overwrite.
   controllerPolicyText = ''
     path "auth/cert/certs/hive-*" {
       capabilities = ["create", "update", "read", "delete"]
@@ -201,7 +209,7 @@ let
     }
 
     path "${credentialMountPath}/data/swarm/agents/*" {
-      capabilities = ["create", "update"]
+      capabilities = ["create", "read", "update"]
     }
   '';
 

nix/module-eval/bao-grants.nix

@@ -182,11 +182,13 @@ let
         && !(lib.hasInfix "sys/policies/acl" s);
     }
     {
-      # 🩸 `read` is load-bearing here and is the one capability neither
-      # sibling has. matrix-ctl's first act is to read this path back and stop
-      # if something is there — that read IS "and only once", so without the
-      # capability every container restart would mint a second access token and
-      # invalidate the hive's.
+      # 🩸 `read` is load-bearing here, and the publisher — the one sibling
+      # that still has no `read` — shows what its absence costs. matrix-ctl's
+      # first act is to read this path back and stop if something is there —
+      # that read IS "and only once", so without the capability every container
+      # restart would mint a second access token and invalidate the hive's.
+      # (The controller holds `read` for the same idempotency reason, on the
+      # agent prefix.)
       name = "matrix-ctl may read back the one path it writes";
       ok =
         let
@@ -473,16 +475,18 @@ let
         && !(lib.hasInfix "path \"secret/*\"" s);
     }
     {
-      # Write-only is the property, not an accident of how it was typed: a
-      # `read` here would let the controller recover every agent's credentials
-      # instead of only replacing them. Pinned as the whole capability list,
-      # because an added capability is exactly what a presence check misses.
-      name = "the controller's grant on agent credentials is write-only";
+      # The exact list is the property, not an accident of how it was typed.
+      # `read` is in it because `mint_and_verify` reads a queue credential back
+      # before rewriting it; `list` is not, so the controller can fetch a
+      # credential only for an agent it was handed the name of, never enumerate
+      # the tree. Pinned as the whole capability list, because an added
+      # capability is exactly what a presence check misses.
+      name = "the controller's grant on agent credentials is create/read/update and nothing else";
       ok =
         let
           s = baoGrantHere.systemd.services.swarm-bao-controller-policy.script;
         in
-        lib.hasInfix "path \"secret/data/swarm/agents/*\" {\n  capabilities = [\"create\", \"update\"]" s;
+        lib.hasInfix "path \"secret/data/swarm/agents/*\" {\n  capabilities = [\"create\", \"read\", \"update\"]" s;
     }
     {
       # The policy above grants paths under a mount nothing else creates, so