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.

hive-agent/src/identity.rs

@@ -31,9 +31,9 @@ pub fn hive_domain() -> Option<String> {
 /// Human display name of this hive (e.g. `pr1ma`). Distinct from
 /// [`hive_domain`] — the domain is the machine-readable DNS address;
 /// this is the prose label humans use in conversation. Returns None
-/// when the host-side `services.hyperhive.hiveName` option is unset,
-/// in which case callers fall back to the domain or the short label
-/// at their discretion.
+/// when `/etc/hyperhive-hive-facts` carries no name (the host's
+/// `services.hyperhive.hiveName` is unset), in which case callers fall
+/// back to the domain or the short label at their discretion.
 #[must_use]
 pub fn hive_name() -> Option<String> {
     non_empty_env("HYPERHIVE_HIVE_NAME")