fix(config): preserve original .env file mode instead of unconditionally tightening to 0600 (#33699)

`save_env_value()` captures the original .env file mode (e.g. 0640 for Docker
volume mounts) and restores it via `os.chmod` — but then unconditionally calls
`_secure_file(env_path)` on the next line, which re-tightens the mode to 0600
and defeats the entire preservation logic. The intent (preserve when
`original_mode` is captured, secure otherwise) was already in the code but
got short-circuited.

Move `_secure_file()` into the `else` branch so it only runs when no original
mode was captured — fresh `.env` files written for the first time still get
the 0600 hardening treatment, but operator-set modes survive subsequent writes.

Salvages #31518 by @blut-agent (config.py portion only). Their PR also bundled
unrelated lowercase-lookup changes in `hermes_cli/commands.py`; this salvage
takes only the focused config fix. The commands.py changes are reasonable on
their own merits but belong in a separate PR.

Co-authored-by: blut-agent <278569635+blut-agent@users.noreply.github.com>

tests/hermes_cli/test_config.py

@@ -292,6 +292,25 @@ class TestSaveEnvValueSecure:
             env_mode = (tmp_path / ".env").stat().st_mode & 0o777
             assert env_mode == 0o600
 
+    def test_save_env_value_preserves_existing_file_mode_on_posix(self, tmp_path):
+        """Regression for #31518: pre-existing .env mode (e.g. 0640 for a
+        Docker bind-mount that the operator chose) survives subsequent
+        writes. Previously _secure_file ran unconditionally after the
+        mode-restore branch and re-tightened to 0600.
+        """
+        if os.name == "nt":
+            return
+
+        env_path = tmp_path / ".env"
+        env_path.write_text("EXISTING=value\n")
+        os.chmod(env_path, 0o640)
+
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            save_env_value("TENOR_API_KEY", "sk-test-secret")
+
+        env_mode = env_path.stat().st_mode & 0o777
+        assert env_mode == 0o640, f"expected 0o640, got {oct(env_mode)}"
+
 
 class TestRemoveEnvValue:
     def test_removes_key_from_env_file(self, tmp_path):