Add claude-railguards to Code Quality & Testing

[lmcdo]

Adds claude-railguards under Code Quality & Testing.

Deterministic pre-write guardrails for coding agents: a PreToolUse hook blocks edits that would duplicate existing code, a pre-implementation gate enforces investigation before database changes, and a QA-report contract gates risky pushes. Distributed as its own Claude Code plugin marketplace (MIT).

• Addresses a real use case: stops an agent silently re-implementing capability that already exists.
• Doesn't duplicate existing entries: block-before-write prior-art guarding isn't covered by the current list.
• Uses an external link to its own marketplace repo, consistent with other externally-linked entries.