Refactor validate_link_inputs() - Split validation concerns

[flyingrobots]

Part of SRP Refactoring (Priority 4: Edge Operations)

Current Problem

validate_link_inputs() in src/cli/link.c combines parsing, validation, and attribution retrieval.

Refactoring Plan

Split into:

• parse_link_type() - Type parsing
• validate_confidence() - Confidence validation
• get_attribution_from_env() - Environment retrieval

Acceptance Criteria

• Each validation step independent
• Parsing separated from validation
• Environment access isolated
• Clear error messages
• Reusable validators
• Easy to test

Technical Notes

• Return structured validation results
• Consider validation pipeline
• Make validators composable

Related to SRP_REFACTORING_PLAN.md Priority 4.2