agents: login shells read the hive facts too; test the readers

Part of #5081. Follow-up to the review of #5131.

- Before this PR, claude read CLAUDE_REMOTE_CONTROL_SESSION_NAME_PREFIX
  from managed-settings.json in every context. The environment generator
  reaches every unit of the container's manager, including `machinectl
  shell` (hivectl choom): machined starts it as a transient
  container-shell@ unit (systemd 260.5 src/machine/machine.c:980-1004),
  and every unit's exec environment starts from the manager environment
  the generators fill (src/core/unit.c:5557, src/core/manager.c:3990-4001).
  Shells no unit starts do not get it: `nixos-container root-login` / `run`
  enter with nsenter + `su -l`, and getty logins go through login(1). So
  hive-facts.nix now also exports the generator's output from
  /etc/set-environment (environment.extraInit).
- The generator's read loop kept a last line only when it ended in a
  newline; it now keeps one without.
- checks.script-test-agent-hive-facts runs the rendered generator and the
  shell snippet (under dash and bash's sh) against sample files: both
  facts, a planted LD_PRELOAD/PATH line, a last line without a newline,
  an empty file and no file.
- identity.rs: hive_name()'s doc names the facts file as the source.

nix/script-tests/agent-hive-facts.nix

@@ -0,0 +1,41 @@
+# `checks.script-test-agent-hive-facts` — runs the two readers of
+# `/etc/hyperhive-hive-facts` in ../agent-modules/hive-facts.nix: the systemd
+# environment generator and the login-shell snippet `/etc/set-environment`
+# sources. The cases are in ./agent-hive-facts.sh.
+#
+# What runs is the rendered generator and snippet. The one edit is the facts
+# file's path, moved under the build directory, and the snippet's reference to
+# the generator, pointed at that edited copy.
+{
+  pkgs,
+  lib,
+  self,
+  nixosSystem,
+}:
+let
+  inherit
+    (import ../module-eval/lib.nix {
+      inherit
+        pkgs
+        lib
+        self
+        nixosSystem
+        ;
+    })
+    agentWith
+    ;
+
+  machine = agentWith { services.hyperhive.agent.user.name = "a1"; };
+in
+pkgs.runCommand "hyperhive-script-test-agent-hive-facts"
+  {
+    GENERATOR =
+      machine.environment.etc."systemd/system-environment-generators/50-hyperhive-hive-facts".source;
+    PROFILE = machine.environment.etc."hyperhive/hive-facts.sh".source;
+    EXTRA_INIT = machine.environment.extraInit;
+    SHELLS = "${pkgs.dash}/bin/dash ${pkgs.bash}/bin/sh";
+  }
+  ''
+    ${pkgs.bash}/bin/bash ${./agent-hive-facts.sh}
+    touch "$out"
+  ''