Community-health files: SECURITY.md, CONTRIBUTING.md, issue/PR templates

[VijitSingh97]

Goal

.github/ has only CODEOWNERS + lint.yml. Mirror pithead's launch-readiness community files.

Proposed

• SECURITY.md — vuln-reporting policy (especially relevant given this compiles from source + tunes the kernel).
• CONTRIBUTING.md — how to run shellcheck/shfmt + the test suite locally.
• .github/ISSUE_TEMPLATE/ + PR template.

Acceptance

• SECURITY.md, CONTRIBUTING.md, and issue + PR templates present and discoverable.