feat: migrate to _async

[glepnir]

migrate to upstream _async module

[Copilot]

Pull Request Overview

This PR refactors the codebase to use an async/await pattern, replacing direct coroutine manipulation with a new async utility module. The changes modernize the asynchronous execution model for formatters and linters.

• Introduces a new guard._async module providing async/await primitives
• Refactors format.lua and lint.lua to use the new async utilities
• Updates test fixtures to reflect command changes

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lua/guard/_async.lua	New async utility module providing run, await, and join functions for coroutine management
lua/guard/format.lua	Refactored to use async/await pattern, split formatter logic into pure/impure functions
lua/guard/lint.lua	Migrated from spawn module to async pattern with new exec_linter function
spec/format_spec.lua	Updated test to use sed instead of cat with modified arguments
Makefile	Added test infrastructure using busted with luarocks

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