schedules: keep projecting a schedule whose definition stops decoding

A reconcile projected only the definitions this build decodes, so after
a rollback an agent's already-projected schedule whose definition a newer
build rewrote was dropped from its key within a minute and stopped
firing. The projection now keeps an agent's stored entries whose id
belongs to an unreadable definition, as they were; it never adds one to
a key that did not hold it. A deleted readable definition still leaves
every key.

docs/swarm/schedules.md

@@ -94,10 +94,11 @@ involved.
 `all`. Agents hold no grant on this bucket. Each change re-reads the key and
 writes against its revision, so a prompt exists for every one of its agents or for
 none of them, and two changes at once don't lose one. The controller leaves a
-prompt it can't decode, such as one a newer build wrote, out of the list and
-out of every agent's key, with a warning in its log. It keeps the prompt in
-the document as it was, after the others, and won't reuse its id. A document
-that isn't an object with a list of `schedules` is an error.
+prompt it can't decode, such as one a newer build wrote, out of the list, with
+a warning in its log. An agent's key that already held the prompt keeps it as
+it was, so it goes on firing, and no other key gets it. The controller keeps
+the prompt in the document as it was, after the others, and won't reuse its
+id. A document that isn't an object with a list of `schedules` is an error.
 
 `agent-schedules` holds one key per agent: the prompts naming that agent,
 without their names or other agents. A key the controller can't decode counts