docs: add OSS health files (CONTRIBUTING, issue templates, PR template, SECURITY)

[PaulGrimshaw]

Context

The repo is missing standard OSS health files that GitHub surfaces and that contributors expect.

Files to add

CONTRIBUTING.md

• Prerequisites (Node, pnpm, Docker for Postgres tests)
• Local setup steps
• How to run tests
• PR process and branching conventions
• How to release (once Changesets is adopted)

.github/ISSUE_TEMPLATE/bug_report.md

Standard bug report template (reproduction steps, expected vs actual, version).

.github/ISSUE_TEMPLATE/feature_request.md

Feature request template.

.github/pull_request_template.md

PR checklist (tests pass, docs updated, changeset added).

SECURITY.md

Minimal: where to report vulnerabilities (e.g. email address or GitHub private advisory).