When HERMES_HOME is unset but ~/.hermes/active_profile names a non-default
profile, any data this process writes lands in the default profile — not the
one the operator expects. Before this change the fallback was silent, so
cross-profile contamination (#18594) was invisible until a user noticed
their memory/state ended up in the wrong place.
Now we emit a one-shot warning to stderr the first time this happens in
a process. No raise — there are 30+ module-level callers of get_hermes_home()
and raising from any of them would brick import. Behavior is otherwise
unchanged; subprocess spawners (systemd template, kanban dispatcher, docker
entrypoint) already propagate HERMES_HOME correctly.
Bypasses logging.getLogger() because this runs before logging is configured
in a significant fraction of callers (module import time).
Refs #18594. Credit to @liuhao1024 for surfacing the silent-fallback case
in PR #18600; we kept the diagnostic signal without the import-time raise.
