Closes#3434.
Agent creation had no way to record which hive an agent runs on.
POST /api/agents now requires a hive, so the roster fetched off
GET /api/hives backs a required SelectField here rather than a
free-text field. Single-hive swarms auto-select their only hive;
multi-hive swarms show a disabled placeholder and force an explicit
choice.
Rebuilt on top of the #3448 form kit (merged after this branch was
originally opened): reuses TextField/SelectField/Button instead of
page-scoped input chrome, and SelectField gains an optional disabled
placeholder option (needed for the loading/empty/multi-hive states
here, generalizes cleanly for future callers). Form goes back to a
column layout per mara's earlier visual feedback on this same page
(two fields of different natural width no longer line up in a row).
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