Add cross-agent policy federation

[lleonardo-franco]

Resolves agentguard-ai/tealtiger#123.

Summary:

• Adds a signed, transport-agnostic PolicyFederation token and child context helper for parent-to-child agent propagation.
• Merges inherited budget ceilings, tool allowlists, data classification ceilings, and revocations into local TealEngine policy with the most restrictive rule winning.
• Adds federation-aware decision metadata plus unit and integration coverage for budget, tool, revocation, and cross-SDK token compatibility.

Validation:

• python3 -m py_compile src/tealtiger/core/engine/policy_federation.py src/tealtiger/core/engine/teal_engine.py tests/core/test_policy_federation.py
• PYTHONPATH=src /private/tmp/tealtiger123-venv/bin/python -m pytest tests/core/test_policy_federation.py -q
• PYTHONPATH=src /private/tmp/tealtiger123-venv/bin/python -m pytest tests/core/test_teal_engine.py tests/core/test_policy_federation.py -q
• PYTHONPATH=src /private/tmp/tealtiger123-venv/bin/python -m pytest -q

Note:

• The full pytest suite passes after installing the package and SDK client extras in a temporary venv; it still prints existing Pydantic deprecation warnings.