Remove imported repository skills

[aryamantodkar]

Summary

• Remove the copied codebase-design, domain-modeling, and tdd skill set.
• Remove skill routing from AGENTS.md and .codex/agents/*.toml.
• Update AGENT_HARNESS.md and CONTRIBUTING.md; remove the .gitignore exceptions.
• Keep the configured reviewer agent.

Why

The repository no longer needs the imported skill set. Keep the harness focused on its configured agents and central routing rules.

Invariants

• AGENTS.md remains the source of routing authority and repository policy.
• The configured four-agent setup remains intact.
• No runtime normalization behavior changes.

Validation

• uv run --locked python scripts/agent/check.py focused --pytest tests/test_docs_routes.py
• Parsed .codex/config.toml and all .codex/agents/*.toml files with Python tomllib.
• Confirmed no references to the removed skills remain; git diff --check passes.