feat: add prompt injection corpus summary

[mattpartida]

Summary

• add a dependency-light prompt-injection fixture corpus summary helper with JSON and Markdown output
• cover manifest count aggregation and Markdown rendering with regression tests
• document the helper in the README, agent-security skill, and detector-quality guide

Test Plan

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

Notes

• TDD RED was verified first: the new corpus-summary tests failed because the helper script did not exist.
• Static scan of added lines found no hardcoded secrets, shell injection, eval/exec, pickle, or SQL string-format patterns.