The hive-side loop landed without anything to converge to: nothing wrote
`$KV.hive-wanted.<hive>`, so in production only the "no key" branch ran.
This is the writer.
`WantedWriter` mirrors `StatusReader` — that module reads what hives report,
this one writes what they are told, so it holds a client rather than a bucket
handle and resolves the store on first use. It shares the status reader's
connection: the controller has exactly one by design, and a second connect
would double the auth-callout traffic and give the two paths independent
reconnect state.
The value under a hive's key is the map of every agent on that hive, so a
plain `put` of a single-agent change would drop a concurrent change to a
different agent, with only one revision of history to not recover from.
Writes are read-modify-write against the entry revision, and only
`WrongLastRevision` / `AlreadyExists` count as a lost race — every other
error returns immediately rather than spinning the retry loop and then
blaming a concurrent writer that never existed.
`apply` is split out and tested because it holds the invariant: declaring
one agent preserves the rest, and a current value that will not decode is an
error rather than a fresh start. Overwriting a document nobody can read
discards every other agent's declaration.
Two routes, no swarmctl verb and no jobq node: `create_agent` needs a graph
because it is multi-step, and one CAS'd write is not.
`build_app` is extracted from `main` in the same change because `main` sat at
exactly the `too_many_lines` limit, so adding an endpoint tripped a lint
about the startup sequence. The route list is the part that grows.
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