feat: Implement Knowledge Graph Schema Linter

Summary

Implement KG schema linter based on design from closed PR #294.

Design Plan

See .docs/plans/kg-schema-linter-design.md for full design document.

Key Features

1. Validate KG markdown files against schema rules
2. Report lint issues with severity, code, and message
3. JSON output for CI/CD integration
4. Skill integration for agentic loop validation

New Crate Structure

crates/terraphim_kg_linter/
├── Cargo.toml
├── src/lib.rs      # Core linting logic
├── src/main.rs     # CLI binary
└── tests/basic.rs  # Integration tests

CLI Usage

cargo run -p terraphim_kg_linter -- --path docs/src/kg --strict

Implementation Timeline

Origin

Extracted from closed PR #294 (85 files, conflicting). Fresh implementation approach.

Start with .docs/plans/kg-schema-linter-design.md and the proposed crates/terraphim_kg_linter structure. Review src/lib.rs, src/main.rs, and tests/basic.rs, then run the documented cargo command against docs/src/kg. Done means markdown files are validated, issues report severity, code, and message, JSON output works for CI, and skill integration is covered.