docs: add contract.yaml field reference cheatsheet

[TianHengZhuang]

Summary

Adds docs/contract-cheatsheet.md - a single-page field reference for the Agent Contract v1.1 schema, as requested in #30.

What's Inside

• Minimal valid contract - the smallest contract.yaml that passes schema validation (just �ersion + system.name)
• Full field reference - every schema field with type, required status, allowed values, and a concrete example snippet:
  • Identity: �ersion, system, �gent, workflow, domain
  • Behavior: capabilities,
    esources, inputs, outputs
  • Guardrails: permissions, constraints, side_effects, �pprovals
  • Infra: dependencies, state,
    ecovery,
    eplay, observability
  • Metadata: �rtifacts, security,
    isk, implementation, lifecycle
• Common patterns - three ready-to-adapt templates:
  • Stateless request-response (simplest)
  • Persistent worker with irreversible side effects + approval gate
  • Scheduled task with cost and rate constraints
• Validation instructions - how to check a contract locally before committing

Design Choices

• Filed under docs/ alongside existing reference material (contract-model-v1.1.md, design-principles.md, etc.)
• Uses the same inline-code-fence style as the rest of the docs
• Cross-links to schemas/v1.1/contract.schema.json for the normative definition
• No new dependencies or build steps - it's pure Markdown

Closes #30.

[TianHengZhuang]

Closing this pull request to free capacity on my side after a long stretch without maintainer feedback. The branch remains on my fork if anyone wants to revisit the change later. Thanks for the project.