Checker seat: the verb input, ahead of B0, so the running indexer knows it when B0's manifests invoke and serve it (issue 1921)

B0's first group check failed because the running indexer, built at this trunk,
refused mesh-build-go (invokes seat:mesh-delivery-checker.input) and
mesh-delivery-checker (serves input), and the check's composition took that
refusal. Publishing the verb first rebuilds the indexer (ADR 0573 §3); B0 then
checks as reviewed. The lines are B0's own, reviewed at 1b073c23.

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

go/catalogue/checker_seat.go

@@ -3,8 +3,8 @@ package catalogue
 // CheckerSeat is the seat mesh-delivery-checker holds.
 const CheckerSeat = "mesh-delivery-checker"
 
-// checkerVerbs are the checker seat's verbs: ask a try, read one, end one by hand. Optional until the checker serves them
-// everywhere.
+// checkerVerbs are the checker seat's verbs: ask a try, read one, end one by hand, and answer a checking program's input.
+// Optional until the checker serves them everywhere.
 func checkerVerbs() []Verb {
 	return []Verb{
 		{Name: "try", Optional: true, Description: "Try a pushed head before its pull request is opened: the checker " +
@@ -28,5 +28,12 @@ func checkerVerbs() []Verb {
 			"id. A try that ended already is refused.",
 			Input: schema(map[string]string{"id": "the try's id, try-<digits>", "why": "why it is ended, in words"},
 				[]string{"id", "why"})},
+		{Name: "input", Optional: true, Description: "A checking program's input the checker keeps, by the sha256 of its " +
+			"exact bytes: the facts of a check its verdict is asked of, or the heads its seat list is asked of, as a run " +
+			"ask names it (its input's verb and digest). Answers the digest and the bytes, base64; a digest it keeps no " +
+			"input of is an error. Asked by the build module that runs the program, as its manifest's invokes grants it. " +
+			"Read-only.",
+			Input: schema(map[string]string{"digest": "the input's sha256, 64 lowercase hex digits, as the run ask names it"},
+				[]string{"digest"})},
 	}
 }