docs: add policy.yaml operator cheatsheet

[CodewithJha]

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Context

Mutiny policies are deterministic tool-use invariants (not LLM judges). Operators live in Core and are exercised in unit tests, but newcomers have no one-page reference for writing policy.yaml.

Goal

Add a short operator cheatsheet under docs/ so a contributor can write or read a policy without digging through Core source.

Why it matters

The sample policy and tests are the real source of truth today. A cheatsheet lowers the barrier for docs PRs, example packs, and first adapters.

Acceptance criteria

• Add docs/POLICY_CHEATSHEET.md listing supported operators / rule kinds with short examples drawn from tests/unit/test_policy_*.py and examples/openai_support_agent/policy.yaml
• Link it from the document map in docs/README.md
• No Core / policy language redesign and no behavior changes
• PR description links this issue

Suggested files

• docs/POLICY_CHEATSHEET.md (new)
• docs/README.md (map link)
• Read-only: tests/unit/test_policy_*.py, examples/openai_support_agent/policy.yaml

Out of scope

• Changing evaluator semantics, adding new operators, or redesigning Hosted policy UI

Hints

• Start from rule kind values (require_args, forbid_args, deny_tool) and constraint ops used in tests (eq, gt, context paths).
• Keep examples copy-paste tiny (3–8 lines each). Prefer tables + fenced YAML.
• After drafting: uv run pytest tests/unit/test_policy_evaluator.py -q should still pass unchanged.

Difficulty

S · docs only · ~half day

Docs

• Catalog: GOOD_FIRST_ISSUES.md #1
• CONTRIBUTING.md
• docs/README.md