Merge main (hq ADR 0266) into the mesh-cli answer, and close what the confirmation review found

- The generic command verb only reads now (commandReads) and terminal-only
  commands are refused through any verb (terminalOnly). mesh-cli's
  ordinary line made neither check: `node account`, `token issue` and
  `secret export` from another node would have run. It now meets both, in
  the one function the command verb shares.
- The serving controller marks itself and its children never the terminal
  (ADR 0266); a line mesh-cli runs as the terminal drops that mark and
  carries MESH_CLI_TERMINAL, so it reads as the terminal it is.
- Two withholding tests searched the answer's text while JSON writes bytes
  as base64, so they held nothing. They search both now, each proved by
  disabling what it guards (Shown, the bus withholding, `calls` via Get).
- The control-node refusal is tested through the assign and unassign acts.

cmd/mesh-controller/sayable_test.go

@@ -29,7 +29,7 @@ func TestMain(m *testing.M) {
 	// The process a mesh-cli test runs as a command line: it says the verb and the caller it was given, and
 	// ends (meshcli_test.go).
 	if os.Getenv(echoEnvironment) != "" {
-		fmt.Printf("verb=%q caller=%q\n", os.Getenv("MESH_VERB"), os.Getenv("MESH_CALLER"))
+		fmt.Printf("verb=%q caller=%q terminal=%v\n", os.Getenv("MESH_VERB"), os.Getenv("MESH_CALLER"), startedAtTheTerminal())
 		os.Exit(0)
 	}
 	conditions.Unsayable = func(o conditions.Observation, field string, r outward.Refusal) {