feat: expose terminal.temp_dir to redirect session temp root off tmpfs

Some Linux distros (notably RAM-based tmpfs /tmp on several Arch-based
setups) cap the temp directory at a small size, so Hermes runs out of
space for session temp files (background logs/pid/exit files, code-
execution sandboxes). Add a terminal.temp_dir config key that points
these at real storage.

- Add terminal.temp_dir default (empty) in config_defaults.py
- Bridge it to TERMINAL_TEMP_DIR via TERMINAL_CONFIG_ENV_MAP
- Honor TERMINAL_TEMP_DIR first in LocalEnvironment.get_temp_dir(),
  falling through to TMPDIR//tmp//gettempdir when unset/invalid
- Add tests covering override, process-env, missing-dir, and empty

hermes_cli/config.py

@@ -3640,6 +3640,7 @@ TERMINAL_CONFIG_ENV_MAP = {
     "modal_mode": "TERMINAL_MODAL_MODE",
     "degraded_mode": "TERMINAL_DEGRADED_MODE",
     "cwd": "TERMINAL_CWD",
+    "temp_dir": "TERMINAL_TEMP_DIR",
     "timeout": "TERMINAL_TIMEOUT",
     "lifetime_seconds": "TERMINAL_LIFETIME_SECONDS",
     "docker_image": "TERMINAL_DOCKER_IMAGE",

hermes_cli/config_defaults.py

@@ -382,6 +382,13 @@ DEFAULT_CONFIG = {
         # preserves the historical error + traceback behavior.
         "degraded_mode": "warn",
         "cwd": ".",  # Use current directory
+        # Root directory for Hermes' terminal session temp files (background
+        # logs/pid/exit files, code-execution sandboxes, etc.). Defaults to the
+        # OS temp root. Set this to a path on real storage when /tmp is a small
+        # tmpfs and Hermes runs out of space — e.g. on RAM-based tmpfs setups
+        # common to some Arch-based distros. Must be an absolute POSIX path;
+        # relative or empty values fall back to the default temp resolution.
+        "temp_dir": "",
         # Terminal font family for the desktop app's embedded xterm.js terminal.
         # When set (e.g. "'CaskaydiaCoveNerdFont', 'JetBrains Mono', monospace"),
         # the desktop terminal uses this as the CSS font-family value, with the

tests/tools/test_local_temp_dir.py

@@ -0,0 +1,60 @@
+"""Tests for ``LocalEnvironment.get_temp_dir`` temp-dir redirect.
+
+Hermes exposes ``terminal.temp_dir`` (mirrored to ``TERMINAL_TEMP_DIR``) so
+users on RAM-based tmpfs ``/tmp`` can point session temp files (background
+logs/pid/exit files, code-execution sandboxes) at real storage.
+"""
+
+import os
+import sys
+
+import pytest
+
+from tools.environments.local import LocalEnvironment
+
+
+def _make_local_env(env: dict) -> LocalEnvironment:
+    """Construct a LocalEnvironment without running init_session (no bash)."""
+    obj = LocalEnvironment.__new__(LocalEnvironment)
+    obj.env = dict(env)
+    return obj
+
+
+def test_temp_dir_override_honored(tmp_path):
+    target = str(tmp_path)
+    env = _make_local_env({"TERMINAL_TEMP_DIR": target})
+    assert env.get_temp_dir() == target
+
+
+def test_temp_dir_from_process_env(tmp_path):
+    target = str(tmp_path)
+    env = _make_local_env({})
+    prev = os.environ.get("TERMINAL_TEMP_DIR")
+    os.environ["TERMINAL_TEMP_DIR"] = target
+    try:
+        assert env.get_temp_dir() == target
+    finally:
+        if prev is None:
+            os.environ.pop("TERMINAL_TEMP_DIR", None)
+        else:
+            os.environ["TERMINAL_TEMP_DIR"] = prev
+
+
+def test_temp_dir_non_existent_falls_through(tmp_path):
+    """A configured path that does not exist must not be used."""
+    missing = str(tmp_path / "does-not-exist")
+    env = _make_local_env({"TERMINAL_TEMP_DIR": missing})
+    # Should fall through to the standard TMPDIR//tmp//gettempdir chain, not
+    # return the missing path.
+    assert env.get_temp_dir() != missing
+
+
+def test_temp_dir_empty_falls_through(tmp_path, monkeypatch):
+    """An empty/relative terminal.temp_dir must not redirect."""
+    monkeypatch.setenv("TMPDIR", str(tmp_path))
+    env = _make_local_env({"TERMINAL_TEMP_DIR": ""})
+    assert env.get_temp_dir() == str(tmp_path)
+
+
+if __name__ == "__main__":
+    sys.exit(pytest.main([__file__, "-q"]))

tools/environments/local.py

@@ -1789,6 +1789,13 @@ class LocalEnvironment(BaseEnvironment):
             # Force forward slashes so the same string serves both contexts.
             return str(cache_dir).replace("\\", "/")
 
+        # Explicit temp-dir override from terminal.temp_dir (TERMINAL_TEMP_DIR).
+        # Honored ahead of the generic TMPDIR so users can redirect Hermes' temp
+        # root to real storage when /tmp is a small tmpfs.
+        configured = self.env.get("TERMINAL_TEMP_DIR") or os.environ.get("TERMINAL_TEMP_DIR")
+        if configured and configured.startswith("/") and os.path.isdir(configured):
+            return configured.rstrip("/") or "/"
+
         for env_var in ("TMPDIR", "TMP", "TEMP"):
             candidate = self.env.get(env_var) or os.environ.get(env_var)
             if candidate and candidate.startswith("/"):