feat: migrate from env vars to config.toml

- Add internal/config package with TOML parsing (BurntSushi/toml)
- Create config.example.toml documenting all settings
- Update main.go to load config via -config flag (default: config.toml)
- Environment variables remain as fallback overrides for backward compat
- Config file values are used as defaults; env vars override when set
- Add comprehensive tests for file loading, defaults, and env overrides
- Add config.toml to .gitignore (secrets stay local)

.gitignore

@@ -1 +1 @@
-.agent/
+config.toml