docs: add limitations sections to skills

[kamalsrini]

Summary

• Adds a Limitations section to all 45 skill entrypoints
• Covers blind spots, false-positive risks, required evidence, and escalation rules
• Normalizes the existing SOC 2 limitations section to the same structure

Linear

• UNI-8

Verification

• rg -l '^## Limitations$' skills///SKILL.md | wc -l
• required limitation bullet count checks for all 45 skills
• ruby scripts/validate_skill_schema.rb
• git diff --check -- skills