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.

hive-c0re/src/socket_server/config_approvals.rs

@@ -38,14 +38,33 @@ pub(super) fn handle_request_init_config(
     // path the `request_init_config` tool takes on every hive. Guarding
     // only the rarer one would have left the common flow exactly as
     // unguarded as before.
-    let warnings = if hive_types::is_reserved_name(name) {
-        tracing::warn!(%agent, %name, "request_init_config: reserved name");
-        vec![format!(
-            "agent name {name:?} is a reserved protocol name — messages from this agent will be \
-             indistinguishable from hyperhive's own; this will become an error"
-        )]
-    } else {
-        Vec::new()
+    //
+    // The blacklist itself comes from nix via `HIVE_RESERVED_NAMES`, so it
+    // stays a config change rather than a rebuild. An UNSET variable means
+    // this daemon was never told — which is not the same as "no name is
+    // reserved", and saying nothing there would be a check that reports
+    // clean because it could not run.
+    let raw = hive_types::reserved_names_raw();
+    let warnings = match raw.as_deref().map(hive_types::parse_reserved_names) {
+        None => {
+            tracing::error!(
+                var = hive_types::RESERVED_NAMES_ENV,
+                "request_init_config: reserved-name check could not run — variable not set"
+            );
+            vec![format!(
+                "the reserved-name check did not run: {} is unset, so {name:?} was accepted \
+                 without being checked against the protocol literals",
+                hive_types::RESERVED_NAMES_ENV
+            )]
+        }
+        Some(reserved) if hive_types::is_reserved_name(name, &reserved) => {
+            tracing::warn!(%agent, %name, "request_init_config: reserved name");
+            vec![format!(
+                "agent name {name:?} is a reserved protocol name — messages from this agent will \
+                 be indistinguishable from hyperhive's own; this will become an error"
+            )]
+        }
+        Some(_) => Vec::new(),
     };
     match submit_init_config(coord, name, Some(agent), description) {
         Ok(_id) if warnings.is_empty() => Response::Ok,