Adds a "Skill Invocation During Plan Mode" section to the preamble resolver so
all generated SKILL.md files include it. Fixes a bug where Claude treats loaded
skill content as reference material instead of executable instructions, and keeps
trying to ExitPlanMode instead of following the skill workflow step by step.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>