fix(constants): warn once when get_hermes_home() falls back under an active profile (#18746)

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.

tests/test_hermes_home_profile_warning.py

@@ -0,0 +1,116 @@
+"""Tests for get_hermes_home() profile-mode fallback warning.
+
+Regression test for https://github.com/NousResearch/hermes-agent/issues/18594.
+
+When HERMES_HOME is unset but an active_profile file indicates a non-default
+profile is active, get_hermes_home() should:
+  1. STILL return ~/.hermes (raising would brick 30+ module-level callers)
+  2. Emit a loud one-shot warning to stderr so operators can diagnose
+     cross-profile data contamination after the fact.
+
+The warning goes to stderr directly (not through logging) because this
+function is called at module-import time from 30+ sites, often before the
+logging subsystem has been configured.
+"""
+
+from pathlib import Path
+
+import pytest
+
+
+@pytest.fixture
+def fresh_constants(monkeypatch, tmp_path):
+    """Import hermes_constants fresh and reset the one-shot warn flag."""
+    import importlib
+    import hermes_constants
+    importlib.reload(hermes_constants)
+    monkeypatch.setattr(Path, "home", lambda: tmp_path)
+    monkeypatch.delenv("HERMES_HOME", raising=False)
+    return hermes_constants
+
+
+class TestGetHermesHomeProfileWarning:
+    def test_classic_mode_no_active_profile_no_warning(
+        self, fresh_constants, tmp_path, capsys
+    ):
+        """Classic mode: no active_profile file → silent, returns ~/.hermes."""
+        result = fresh_constants.get_hermes_home()
+        assert result == tmp_path / ".hermes"
+        assert "HERMES_HOME fallback" not in capsys.readouterr().err
+
+    def test_default_active_profile_no_warning(
+        self, fresh_constants, tmp_path, capsys
+    ):
+        """active_profile=default → still no warning, returns ~/.hermes."""
+        hermes_dir = tmp_path / ".hermes"
+        hermes_dir.mkdir()
+        (hermes_dir / "active_profile").write_text("default\n")
+        result = fresh_constants.get_hermes_home()
+        assert result == tmp_path / ".hermes"
+        assert "HERMES_HOME fallback" not in capsys.readouterr().err
+
+    def test_named_profile_unset_home_warns_once(
+        self, fresh_constants, tmp_path, capsys
+    ):
+        """active_profile=coder + HERMES_HOME unset → warn loudly, still return fallback."""
+        hermes_dir = tmp_path / ".hermes"
+        hermes_dir.mkdir()
+        (hermes_dir / "active_profile").write_text("coder\n")
+
+        result = fresh_constants.get_hermes_home()
+
+        # 1. Still returns the fallback — no import-time crash
+        assert result == tmp_path / ".hermes"
+        # 2. Stderr got the warning exactly once
+        err = capsys.readouterr().err
+        assert err.count("HERMES_HOME fallback") == 1
+        assert "'coder'" in err
+        assert "#18594" in err
+
+        # 3. One-shot: second and third calls don't re-warn
+        fresh_constants.get_hermes_home()
+        fresh_constants.get_hermes_home()
+        err2 = capsys.readouterr().err
+        assert "HERMES_HOME fallback" not in err2
+
+    def test_hermes_home_set_suppresses_warning(
+        self, fresh_constants, tmp_path, capsys, monkeypatch
+    ):
+        """Even if active_profile is 'coder', setting HERMES_HOME suppresses warning."""
+        profile_dir = tmp_path / ".hermes" / "profiles" / "coder"
+        profile_dir.mkdir(parents=True)
+        (tmp_path / ".hermes" / "active_profile").write_text("coder\n")
+        monkeypatch.setenv("HERMES_HOME", str(profile_dir))
+
+        result = fresh_constants.get_hermes_home()
+
+        assert result == profile_dir
+        assert "HERMES_HOME fallback" not in capsys.readouterr().err
+
+    def test_unreadable_active_profile_no_crash(
+        self, fresh_constants, tmp_path, capsys
+    ):
+        """active_profile that can't be decoded → fall through silently."""
+        hermes_dir = tmp_path / ".hermes"
+        hermes_dir.mkdir()
+        # Write bytes that aren't valid utf-8
+        (hermes_dir / "active_profile").write_bytes(b"\xff\xfe\x00\x00")
+
+        result = fresh_constants.get_hermes_home()
+
+        assert result == tmp_path / ".hermes"
+        # Shouldn't crash; shouldn't warn either (can't tell what profile was intended)
+        assert "HERMES_HOME fallback" not in capsys.readouterr().err
+
+    def test_empty_active_profile_no_warning(
+        self, fresh_constants, tmp_path, capsys
+    ):
+        """Empty active_profile file → treated as default, no warning."""
+        hermes_dir = tmp_path / ".hermes"
+        hermes_dir.mkdir()
+        (hermes_dir / "active_profile").write_text("")
+
+        result = fresh_constants.get_hermes_home()
+
+        assert result == tmp_path / ".hermes"
+        assert "HERMES_HOME fallback" not in capsys.readouterr().err