A sync that changed nothing says so again

Comparing the per-node digest against an engine that does not record one is
comparing against nothing, and reporting every node as changed on every sync
for ever — which is what a client newer than its engine did, since `NodeDef`
drops fields it has never heard of. A node is named now only when both sides
carry a digest, so a no-op sync is `unchanged` again and the signal one syncs
for is back.

That silence had also been the only sign of the mismatch, so sync now names
it: one line saying the engine stored no record of what a node's code reaches,
with both versions in it and what to run. Bumped to 0.1.6 — the digest changed
the stored document's shape, and a version that does not move makes two
different engines indistinguishable, which is the thing it was made
load-bearing for a day ago.

`— draft` was printed whenever there was simply nothing to publish, which
reads as work left unfinished. It is said only when a draft is genuinely
there, and `— published` when one was.

docs/code/cli.md

@@ -167,7 +167,13 @@ its way to, and what they hash to. That is what the
 [stage cache](../concepts/runs.md#stage-caching) keys on, so editing a
 helper a node calls into is reported as that node changing — `train: updated
 (flow, fit)` — and re-runs it, while editing something the node never reaches
-is left alone. See
+is left alone.
+
+A sync that changed nothing says `unchanged`, which is the answer worth
+having. `— published` and `— draft` are said only when there was something to
+publish or a draft was genuinely left behind. An engine too old to store what
+a node's code reaches says so in a line naming both versions; until it is
+upgraded its cache is keyed on the whole repository, as it was before. See
 [Getting started: data science](../getting-started/data-science.md).
 
 ### `fluksio run`