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/hive-c0re/environment.nix

@@ -143,6 +143,21 @@ in
   # mandatory, so this is unconditional (the whole env block is already
   # gated on hyperhive being enabled). See `docs/gateway.md::HIVE_FORGE_URL`.
   HIVE_FORGE_URL = "http://${config.services.hyperhive.swarm.forge.domain}";
+
+  # The one blacklist of names an agent may not take — see
+  # `nix/reserved-names.nix`, which is also read by the swarm controller, by
+  # the swarm collector's owner assertion, and by the test suite. Nix owns it
+  # so that keeping it current is a config change, not a rebuild of a binary.
+  #
+  # Whitespace-separated rather than JSON: every entry is an `Ident`
+  # (`[a-z0-9-]`), so a space can never occur inside a name and the encoding
+  # cannot be lossy. Spelling it as JSON would put a parser in the crate whose
+  # whole point is to have no dependencies.
+  #
+  # Unconditional on purpose. The consumer treats an ABSENT variable as "I was
+  # never told" and says so out loud, which is the correct reading — but it is
+  # a reading no correctly-built hive should ever have to make.
+  HIVE_RESERVED_NAMES = lib.concatStringsSep " " (import ../../reserved-names.nix);
 }
 //
   lib.optionalAttrs