fix(config): Include explain level default

[Gujiassh]

Summary

• Add explain_level to the canonical gstack-config defaults table
• Include explain_level in list and defaults output
• Update focused config tests so unset explain_level reports the documented default

Fixes #1607.

Verification

• bun test test/explain-level-config.test.ts
• git diff --check

[jbetala7]

Collision note: #1608 is already open for the same #1607 fix. It touches the same bin/gstack-config default/list/defaults behavior, links the same issue, and has focused coverage for the documented default path.

I would treat #1608 as the canonical/older PR rather than landing a parallel explain_level fix. If there is a useful assertion in this branch that #1608 lacks, the smallest path is to fold that assertion into the canonical branch instead of keeping two PRs for the same user-visible bug.

[Gujiassh]

Closing this as duplicate of the older canonical PR #1608 for the same #1607 fix. Thanks for catching the overlap; I should not keep a parallel explain_level branch open.