docs: move cross-layer contracts to organization

[swanchristmas]

What changed

• remove the duplicate cross-layer docs/agentic-tooling.md
• link the organization-owned tooling model from the README and harness docs
• narrow the local roadmap to rules-layer reliability and evidence
• route shared evaluation policy and future hooks to the organization roadmap

Why

The harness repository owns the rules layer, not the philosophy or roadmap that coordinates rules, skills, and future hooks. Those contracts now live in agentic-tend/.github.

Validation

• git diff --check
• all Markdown files remain below 100 lines
• uv run --with copier --with pytest --with pyyaml -- pytest tests/ -q — 9 passed