docs(chapter-02): add the community scaffolding for contributors

Chapter: 02 - Open-source collaboration.

Plain-English map:
Add a code of conduct, security policy, issue templates, and a pull request
template. These files tell people how to report bugs, propose changes, and
disclose vulnerabilities without guessing.

Why it matters:
Open-source projects are not only code. They need a shared operating manual
so first-time contributors know the rules of the room and maintainers get the
information they need.

Principle:
Healthy collaboration is documented. A project should make the safe path the
obvious path, especially for security reports.

Precedent borrowed:
GitHub community-health conventions and the security-reporting guidance
requested in upstream PR #147.

Upstream base: willchen96/mike@d39f580.
Original local commit: 045bdb7.