Deliver BAT-003: daemon .env loading (REQ-010, minor-change)

DaemonConfig.from_env() now best-effort loads a .env file (find_dotenv
usecwd, load_dotenv override=False) so operators can manage daemon env
vars via .env, with real OS env taking precedence. Add python-dotenv
dependency, gitignore .env, ship .env.example, and add config tests
(load / OS-env precedence / absence regression / missing-DSN guard).

Verification: 78 passed, ruff clean, .env ignored + .env.example tracked,
and a .env-only DaemonConfig smoke. Processed via the minor-change fast
path: REQ-010 (Configuration) -> Implemented, BAT-003 (batch-lite,
no design) -> release-candidate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/runtime/test_config_dotenv.py

@@ -0,0 +1,50 @@
+"""DaemonConfig .env loading tests (REQ-010)."""
+from __future__ import annotations
+
+import pytest
+
+from trex_core.runtime.config import DaemonConfig
+
+_KEYS = ("TREX_POSTGRESQL_DSN", "TREX_TOTAL_SLOTS", "TREX_API_HOST", "TREX_API_PORT")
+
+
+@pytest.fixture(autouse=True)
+def _clean_env(monkeypatch):
+    for k in _KEYS:
+        monkeypatch.delenv(k, raising=False)
+
+
+def test_loads_values_from_dotenv(tmp_path, monkeypatch):
+    """AC-1: .env in CWD is loaded when the OS env var is unset."""
+    (tmp_path / ".env").write_text(
+        "TREX_POSTGRESQL_DSN=dbname=from_dotenv\nTREX_TOTAL_SLOTS=7\n"
+    )
+    monkeypatch.chdir(tmp_path)
+    cfg = DaemonConfig.from_env()
+    assert cfg.dsn == "dbname=from_dotenv"
+    assert cfg.total_slots == 7
+
+
+def test_os_env_takes_precedence_over_dotenv(tmp_path, monkeypatch):
+    """AC-2: real OS env wins over .env (override=False)."""
+    (tmp_path / ".env").write_text("TREX_POSTGRESQL_DSN=dbname=from_dotenv\n")
+    monkeypatch.chdir(tmp_path)
+    monkeypatch.setenv("TREX_POSTGRESQL_DSN", "dbname=from_os_env")
+    cfg = DaemonConfig.from_env()
+    assert cfg.dsn == "dbname=from_os_env"
+
+
+def test_no_dotenv_uses_os_env_only(tmp_path, monkeypatch):
+    """AC-3: with no .env, behaviour is unchanged (os.environ only)."""
+    monkeypatch.chdir(tmp_path)  # empty dir, no .env
+    monkeypatch.setenv("TREX_POSTGRESQL_DSN", "dbname=os_only")
+    cfg = DaemonConfig.from_env()
+    assert cfg.dsn == "dbname=os_only"
+    assert cfg.total_slots == 10  # default
+
+
+def test_missing_dsn_raises(tmp_path, monkeypatch):
+    """No .env and no env var -> required DSN error (regression guard)."""
+    monkeypatch.chdir(tmp_path)
+    with pytest.raises(RuntimeError):
+        DaemonConfig.from_env()