feat: Add configuration loader with multi-format support

[Nexussyn]

Summary

Implements a configuration loader module that supports:

• Multiple file formats (TOML, YAML, JSON)
• Environment variable overrides
• Validation and default values
• Type-safe configuration structs

Closes #18

[Nexussyn]

✅ Ready for merge — All issues resolved:

• Restored all original Cargo.toml dependencies (tokio, tracing, uuid, clap, anyhow, etc.) that were accidentally dropped
• Added serde_yaml = "0.9" as new dependency for YAML config support
• Kept tempfile = "3" in [dev-dependencies]
• Config multi-format loader implementation is clean and complete with full test coverage

This PR is ready for review and merge.