feat: add local environment diagnostics

[cayasde]

Summary

• add lutest doctor with an optional config path
• report local Lute and Roblox runtime readiness without running tests or opening a remote session
• check credentials and bundle preparation internally without exposing dependency details or secrets

Why

• gives users a fast preflight check for configuration and local prerequisites

Validation

• mise run check
• lutest test (132 passing)
• generated binary: lutest doctor

Release

• Changeset included: minor