gstack-config omits explain_level from defaults and active values

[jbetala7]

Observed problem

bin/gstack-config documents explain_level: default in the generated config header and validates explain_level values in set, but the key is missing from the default table and from the list/defaults key enumerations.

Current behavior on upstream main

From current origin/main, in a clean GSTACK_HOME:

GSTACK_HOME=$(mktemp -d) git show origin/main:bin/gstack-config | bash -s -- get explain_level
# prints an empty string

GSTACK_HOME=$(mktemp -d) git show origin/main:bin/gstack-config | bash -s -- defaults | grep explain_level
# no output

GSTACK_HOME=$(mktemp -d) git show origin/main:bin/gstack-config | bash -s -- list | grep explain_level
# no output

By comparison, a documented key such as update_check returns its default value.

Expected behavior

gstack-config get explain_level should return default when the user has not set a value, and gstack-config list / gstack-config defaults should include explain_level alongside the other documented config keys.

Duplicate searches performed

I searched open issues and PRs for:

• explain_level
• gstack-config explain_level
• gstack-config get explain_level
• writing style config
• config defaults
• bin/gstack-config

The only nearby open config PR I found is #1514, which adds learnings_preamble_limit and learnings_skill_limit; it does not address explain_level.

Candidate fix shape

Add explain_level) echo "default" ;; to lookup_default, include explain_level in the list and defaults key enumerations, and add a focused regression test for get, defaults, and list.

[jbetala7]

Opened PR #1608 with the focused config default fix: #1608