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>

.env.example

@@ -0,0 +1,10 @@
+# trex-core slot-core daemon configuration (copy to .env; .env is gitignored).
+# Real OS environment variables take precedence over values in .env.
+
+# Required: psycopg DSN for the PostgreSQL single-writer store.
+TREX_POSTGRESQL_DSN=dbname=trex_core host=/var/run/postgresql
+
+# Optional (defaults shown).
+TREX_TOTAL_SLOTS=10
+TREX_API_HOST=127.0.0.1
+TREX_API_PORT=8080