chore: prepare protocol for public release

[TheusHen]

Summary

• add public contributor, conduct, support, and vulnerability-reporting guidance
• add issue and pull-request templates plus credential-oriented ignore rules
• add Dependabot and a CodeQL workflow that activates after public visibility
• document the remaining GitHub settings required immediately before the visibility change

Validation

• cd implementations/go && go vet ./... && go test ./...
• python conformance/runner/run.py

Follow-up before changing visibility

• Enable private vulnerability reporting, Dependabot alerts, secret scanning, push protection, Code Scanning, and main branch protection as documented in PUBLICATION.md.
• Review the existing commit-author address and decide whether it is intended to be public; history rewriting is deliberately out of scope for this PR.