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/checks.nix

@@ -132,6 +132,12 @@ in
     inherit pkgs self nixosSystem;
     inherit (pkgs) lib;
   };
+  # Runs ./agent-modules/hive-facts.nix's generator and login-shell snippet
+  # against sample facts files.
+  script-test-agent-hive-facts = import ./script-tests/agent-hive-facts.nix {
+    inherit pkgs self nixosSystem;
+    inherit (pkgs) lib;
+  };
   module-eval-agent-github-bao = import ./module-eval/agent-github-bao.nix {
     inherit pkgs self nixosSystem;
     inherit (pkgs) lib;