fix(doctor): report missing discovery roots

[cayasde]

Reproduction

Create empty/lutest.toml:

[discovery.lute]
require = "@lutest_release"
roots = ["missing"]

Run:

lutest doctor --config empty/lutest.toml

Actual behavior

The command exits with code 0 and reports:

+ Lute runtime ready
  roots: missing

A subsequent lutest test --config empty/lutest.toml fails because the configured root does not exist.

Expected behavior

doctor should not report a runtime as ready when a configured literal discovery root is missing. It should report the unavailable root and exit with code 1. Glob roots may need a separate validation rule.