feat: define EACS 0.1 executable case contract

[Saitamasans]

EACS 0.1 contract layer

• Adds ADR 0001, EACS and validation-report JSON Schemas, deterministic validator, and multi-api cases validate FILE.
• Adds valid single API, multi-API, and review-required examples plus schema, YAML/JSON CLI, variable, secret-masking, dependency, assertion, and maintenance-key tests.
• Does not change ATIR, Runner request/assertion/DNS/risk/evidence logic, package version 0.1.2, tags, or releases.

Not implemented

• EACS to ATIR compilation
• EACS run, preview, diff, or backfill
• testing-skills dual output

Validation

• ruff format --check .
• ruff check .
• pytest -q (162 passed, 3 skipped)
• pytest --cov=multi_api_executor --cov-branch (83%)
• schema, skill, security validation

Draft only; awaiting CI and human EACS schema review.