fix(compress): make validation catch what it promised, and stop the fence strip eating documents

strip_llm_wrapper matched any document that merely started and ended with a
fence line, so an ordinary README section came back with both markers deleted
and its code blocks merged into prose. It now requires the two fences to be the
same block.

The validator gains CommonMark indented code blocks — a 4-space-indented
destructive command was prose to it, so a rewritten `kubectl delete` passed
clean and overwrote the user's file. Lost file paths and changed heading text
are errors now, not warnings; both were promised in SKILL.md and neither gated
the overwrite.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SqmbqWDtNexzDfH5V3BMRz