fix: keep internal section numbers out of runtime messages

Four messages that reach an operator or an API client cited sections
of a specification nobody outside the project can read:

- the unimplemented archive store error from config load
- the running-server cap refusal, from both the user wake and the
  internal wake
- the missing memory ceiling guard, in the API and in felis apply

The references are gone and the wording is otherwise unchanged. Each
message still says what went wrong and, where there is one, what to
do about it. The test for the archive store message checks for the
tarLocal remediation, which is still there.

internal/api/handlers_internal.go

@@ -149,7 +149,7 @@ func (a *API) handleInternalWake(w http.ResponseWriter, r *http.Request) {
 	}
 	if !ok {
 		writeError(w, r, newError(http.StatusServiceUnavailable, "at_capacity",
-			"the cluster is at its running-server cap (spec §9.1); retry once a server stops"))
+			"the cluster is at its running-server cap; retry once a server stops"))
 		return
 	}