Adds explain_level: default|terse to the annotated config header with
a one-line description. Whitelists valid values; on set of an unknown
value, prints a specific warning ("explain_level '\$VALUE' not
recognized. Valid values: default, terse. Using default.") and writes
the default value. Matches V1 preamble's EXPLAIN_LEVEL echo expectation.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>