docs(rebrand): R3 — rewrite community files for plane-ce-mcp (#4)

- CONTRIBUTING: CE-focused contribution guide (issue template hints,
  dev setup, tool rules from compat layer)
- SECURITY: GitHub Security Advisories on this repo; Plane-itself issues
  routed to makeplane's policy
- CODE_OF_CONDUCT: contact swapped from squawk@plane.so to this repo

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>