Add HTTP parser-boundary gates to secure-code-review

[KooZuKi]

Summary

• add HTTP parser-boundary and request smuggling review guidance
• add CL/TE conflict, duplicate-header, HTTP/2 downgrade, proxy/backend, serverless adapter, and webhook raw-body evidence gates
• update report fields, CWE mapping, pitfalls, references, reviewer template, and changelog

Validation

• git diff --check
• local frontmatter required-field check
• local prompt-injection pattern scan

Bounty

Addresses #1174. Preferred payment details can be provided privately after acceptance.