Add signal detector request issue template

[EtnDiaz]

Summary

• Add a dedicated GitHub issue form for MDVP signal detector requests.
• Capture public examples, rendered DOM/CSS evidence, expected detector behavior, false-positive risk, and contributor readiness.
• Add a static regression test so the signal request form keeps its core fields and existing signal label.

GitLab coordination: https://gitlab.com/tixo-digital/program/-/work_items/143

Verification

• node --test test/issue-templates.test.mjs
• devbox run verify
• devbox run smoke
• PUPPETEER_EXECUTABLE_PATH='/Applications/Google Chrome.app/Contents/MacOS/Google Chrome' devbox run audit-smoke

Notes

• No CLI command semantics changed.
• npm pack --dry-run kept .github/ out of the package and included cli.mjs plus engine/.