test: add prompt injection fixture corpus

[mattpartida]

Summary

• add a manifest-backed prompt-injection fixture corpus covering benign, direct, indirect, encoded, and high-risk config cases
• add regression tests that run the existing signal and exposure scanners against the fixture corpus
• document the corpus in the README repository layout

Test Plan

• python -m compileall -q skills tests
• python -m pytest -q
• ruff check .
• ./package-skills.sh
• git diff --check --cached