feat(rubric): add ReasonCode type for categorized finding codes

Add ReasonCode type with standardized finding identifiers:
- Category prefixes: REQ, METRIC, USER, ARCH, SEC, SCALE, INFRA, DOC, SCOPE, UX
- Pre-defined codes for common issues (e.g., REQ-AMBIGUOUS, SEC-MISSING_AUTH)

Includes ReasonCodeRegistry with metadata:
- DefaultSeverity for each code
- RepairPrompt for AI-assisted remediation

Enables automated repair workflows by providing machine-readable
categorization of issues that map to specific repair strategies.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: grokify