fix(s06): move extract_text() next to its only caller

extract_text() is new in s06 (listed in the docstring under "Changes
from s05") but was defined under the "FROM s02-s05 (unchanged)" banner.
Move it into the "NEW in s06: Subagent" section, directly above its only
caller spawn_subagent(). Pure move, no logic change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>