ci: add automated tests and safe command execution

[mm4cN]

Closes #3

Summary

• add GitHub Actions checks for formatting and tests
• add a headless Neovim test harness using mini.test
• add a plugin loading smoke test
• extract deterministic Conan command builders with test coverage
• execute Conan commands using argv instead of shell strings
• document local development commands

Verification

• StyLua formatting check
• mini.test suite
• git diff --check