types: let nix own the reserved-name blacklist

One list, in nix/reserved-names.nix, handed to everything that needs it
as HIVE_RESERVED_NAMES. Keeping it current becomes a config change
rather than a rebuild, and hive names and agent names -- one namespace
going forward -- are checked against the same file: swarm-otel.nix's
hand-written reservedOwners is gone.

Whitespace-separated rather than JSON, deliberately, unlike the
structured env vars beside it. Every entry is an Ident ([a-z0-9-]), so
whitespace cannot occur inside a name and the encoding is provably
lossless; JSON would mean either a parser dependency in a crate whose
purpose is to have none, or a copy of the parse in every consumer.

An UNSET variable is not "nothing is reserved". Both creation sites log
an error and return a warning saying the check did not run, so a
misconfigured deployment says so instead of silently accepting every
name. A blank value folds into unset: nix always renders a non-empty
list, so present-but-empty is a rendering fault, not a declaration.

Two guards whose subject moved out of their own file now assert their
own case is still in it, because a guard that can be retired by an edit
elsewhere is not a guard:

- swarm-otel.nix asserts reserved-names.nix still contains its
  swarmTierName.
- hive-sh4re's sentinel drift test PANICS when the variable is missing
  rather than skipping -- a drift test that quietly does nothing still
  reports green. checks.nix and devshell.nix both export it so CI and a
  local cargo test agree. Verified as a pair: with the variable set, 8
  tests pass; with it unset, exactly the 4 drift tests fail and the
  unrelated ones still pass.

nix/host-modules/swarm-controller.nix

@@ -648,6 +648,13 @@ in
         # `swarm.peerHives`, `swarm.hives` minus this hive) rather than
         # peers-minus-self. Consumed by `GET /api/hives`
         # (swarm-controller/src/main.rs::load_hives).
+        # The one blacklist of names an agent may not take, shared verbatim
+        # with hive-c0re and with the collector's owner assertion — see
+        # `nix/reserved-names.nix`. Whitespace-separated rather than JSON
+        # like its neighbour below, because every entry is an `Ident` and so
+        # cannot contain a space; the neighbour carries objects and has no
+        # such option.
+        HIVE_RESERVED_NAMES = lib.concatStringsSep " " (import ../reserved-names.nix);
         SWARM_CONTROLLER_HIVES = builtins.toJSON (
           lib.mapAttrsToList (name: h: {
             inherit name;