chore(repo): add contribution infrastructure

[connorcarpenter15]

Summary

• add structured bug and protocol-change issue forms plus private security routing
• add a protocol-aware pull request checklist
• add Markdown lint and link-check CI with locally reproducible configuration
• add shared editor settings and resolve the existing Markdown lint backlog
• keep Buf compatibility breaking checks disabled

Validation

• buf build
• buf lint
• markdownlint-cli2 v0.23.0: 0 errors
• actionlint v1.7.12
• Prettier 3.6.2 YAML parse/format check
• Lychee 0.24.2: 55 links checked, 0 errors