Feature: Add vix tests command

Description

Introduce a new CLI command vix tests [path] to run project tests more explicitly.

This command acts as an alias of:
vix check --tests

Motivation

• Improve discoverability of test execution
• Provide a clearer mental model for users
• Align with other modern CLIs (cargo test, npm test, etc.)

Acceptance criteria

• vix tests executes the same flow as vix check --tests
• Correct exit codes on test success/failure
• Included in vix help