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>

trex_core/runtime/config.py

@@ -1,10 +1,24 @@
-"""Daemon configuration resolution (REQ-005)."""
+"""Daemon configuration resolution (REQ-005, REQ-010)."""
 from __future__ import annotations
 
 import os
 from dataclasses import dataclass
 
 
+def _load_dotenv() -> None:
+    """Best-effort load of a .env file into os.environ (REQ-010).
+
+    Real OS environment variables take precedence over .env (override=False),
+    matching 12-factor expectations. If python-dotenv is not installed or no
+    .env exists, this is a no-op (no regression).
+    """
+    try:
+        from dotenv import find_dotenv, load_dotenv
+    except ImportError:  # pragma: no cover - optional at runtime
+        return
+    load_dotenv(find_dotenv(usecwd=True), override=False)
+
+
 @dataclass(frozen=True)
 class DaemonConfig:
     dsn: str
@@ -14,6 +28,7 @@ class DaemonConfig:
 
     @staticmethod
     def from_env() -> "DaemonConfig":
+        _load_dotenv()
         dsn = os.environ.get("TREX_POSTGRESQL_DSN")
         if not dsn:
             raise RuntimeError(