docs: fix documentation accuracy issues in skills (#157)

- Fix broken commit message HEREDOC syntax in sharing-skills/SKILL.md
  - Move entire message inside command substitution for valid bash

- Fix contradictory test requirements in systematic-debugging/SKILL.md
  - Clarify automated tests are "strongly preferred"
  - One-off scripts should be documented for regression testing
  - Requirement is for "reproducible test" not escape clause

- Fix typo in testing-skills-with-subagents/SKILL.md
  - "ith" -> "with" in checklist item

skills/writing-skills/testing-skills-with-subagents.md

@@ -324,7 +324,7 @@ Before deploying skill, verify you followed RED-GREEN-REFACTOR:
 - [ ] Added explicit counters for each loophole
 - [ ] Updated rationalization table
 - [ ] Updated red flags list
-- [ ] Updated description ith violation symptoms
+- [ ] Updated description with violation symptoms
 - [ ] Re-tested - agent still complies
 - [ ] Meta-tested to verify clarity
 - [ ] Agent follows rule under maximum pressure