Add shared runtime quota policy

[NavinAgrawal]

Summary

• add pure P3 quota and cost-control policy contract
• enforce fail-closed global, team, user, upstream, and tool scope decisions
• document quota guardrails and extend shared-runtime docs contract

Verification

• private: make precommit PYTEST_WORKERS=0 (1411 unit, 152 journey, 0 failed, 0 skipped)
• private: make maintainer-violations PYTEST_WORKERS=0 (0 violations, 12 advisories)
• private: make maintainer-grade-quality PYTEST_WORKERS=0 (A+ 100.0)
• public: make precommit PYTEST_WORKERS=0 (1411 unit, 118 journey, 0 failed, 0 skipped)
• public: make maintainer-violations PYTEST_WORKERS=0 (0 violations, 12 advisories)
• public: make maintainer-grade-quality PYTEST_WORKERS=0 (A+ 100.0)

Mutation was not run locally to respect Mac load governance.