feat: application configuration with Typesafe Config

[vim89]

Add HOCON-based application configuration using Typesafe Config.

Features:

• Process-scoped configuration loaded lazily on first access
• Auto-loads application.conf from classpath
• Environment profile support via CASK_ENV (dev/test/prod)
• Type-safe accessors with Either-based error handling
• Environment variable overrides using ${?VAR} syntax

Changes:

• Add com.typesafe:config:1.4.3 dependency
• Add cask.Config API with functional error types
• Add config example demonstrating usage
• Add comprehensive test suite

Addresses configuration aspect of #181