Integration test suite

[tale]

The current test coverage is a single test/rootbeer.lua smoke test. For a public release we need proper integration tests:

• Core primitives — rb.file, rb.link_file, rb.link, rb.exec, rb.path_exists etc. should all have tests that run against a temp directory and verify the resulting filesystem state.
• Module tests — each stdlib module (zsh, git, ssh, brew, mac) should have at least one test that calls .config() and asserts the generated file content is correct.
• Plan/execute tests — verify that dry-run produces the expected ops without side effects, and that apply actually creates files/links.
• Error cases — missing files, invalid config shapes, permission errors.
• Profile system — test profile.config, profile.select, profile.when with various profile values.

Tests should be runnable via cargo test and not require any special system state.