Say how the spawned tools are actually invoked

Plugin/unraid/Tools/node_chat_receive.php

@@ -5,12 +5,25 @@
 //   vv_nc_deliver(), with the record on stdin.
 //
 // OPERATIONAL MODEL
-//   php node_chat_receive.php --channel=<id>          record on stdin, one JSON object
-//   php node_chat_receive.php --flush                 retry this host's own undelivered spool
-//
 //   Exit 0 means stored. The sender treats anything else as undelivered and spools for retry, so
 //   a non-zero exit here is a message that will arrive later rather than one that is lost.
 //
+//   Two jobs in one file because they are two halves of the same contract: --channel takes what
+//   a partner is delivering now, --flush pushes what this host failed to deliver earlier. A node
+//   is both a receiver and a sender, and splitting them would mean two files that must agree on
+//   the same spool layout.
+//
+// DESIGN PRINCIPLES
+//   The exit code is the receipt, and it is the only one.
+//       There is no acknowledgement message and no reply body. The sender is an SSH command that
+//       already has an exit status, so inventing a second channel to say the same thing would
+//       give the two ways to disagree.
+//
+//   Refusing is cheaper than being wrong.
+//       An unparseable record, an unknown channel or a failed write all exit non-zero and leave
+//       the message in the sender's spool. A retry costs one SSH round trip; a message accepted
+//       and dropped is gone with nothing recording that it existed.
+//
 // OPERATIONAL SAFEGUARDS
 //   Reached only over SSH with a key this mesh installed, so the caller already has root. This
 //   file therefore validates shape, not authority — there is no privilege here to protect that
@@ -22,6 +35,12 @@
 //   Storage is append-and-trim through vv_nc_append(), which is idempotent on message id — a
 //   retry of something that already landed is a no-op rather than a duplicate.
 //
+// RUNTIME MODES
+//   Not invoked by hand — the sending node's vv_nc_deliver() runs it over SSH.
+//
+//   php node_chat_receive.php --channel=<id>   store one JSON record, read from stdin
+//   php node_chat_receive.php --flush          retry this host's own undelivered spool
+//
 // DEPENDS ON
 //   include/node_chat.php   vv_nc_append(), vv_nc_channel(), vv_nc_flush_spool()
 // ═══════════════════════════════════════════════════════════════════════════════════════════════