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.
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