feat(config): split monolithic config + extract secrets

- Update .gitignore for v0.3 multi-file layout
- Add config/.env.example template for secrets
- Add config/local.yaml.example for operator values
- Wire main.py to use new config_loader
- Support both legacy and new layouts

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

config/.env.example

@@ -0,0 +1,19 @@
+# MeshAI Secrets Template
+# Copy to /data/secrets/.env and fill in your values
+# This file is gitignored - never commit real secrets
+
+# LLM API Keys (only one needed based on your backend choice)
+OPENAI_API_KEY=
+ANTHROPIC_API_KEY=
+GOOGLE_API_KEY=
+
+# Mesh Source Credentials
+MESHMONITOR_API_TOKEN=
+MQTT_PASSWORD=
+
+# Environmental Feed Keys
+TOMTOM_API_KEY=
+FIRMS_MAP_KEY=
+
+# Notification Credentials
+SMTP_PASSWORD=