Agent Skills: convert-elixir-roc

Bidirectional conversion between Elixir and Roc. Use when migrating projects between these languages in either direction. Extends meta-convert-dev with Elixir↔Roc specific patterns. Use when migrating Elixir projects to Roc, translating BEAM/OTP patterns to platform-based architecture, or refactoring Elixir codebases. Extends meta-convert-dev with Elixir-to-Roc specific patterns.