feat(integrations): add shared runtime primitives

[sinewaveai]

What

• Adds the MCP runtime integrations PRD for the v0.3-v0.6 integration roadmap.
• Introduces prooflayer.integrations.common shared primitives for future runtime integrations: config, envelopes, decisions, adapter protocol, runtime proxy, tool events, and audit event hashing.
• Refactors the existing LangGraph integration internally to use the shared config/session/audit primitives while preserving the public LangGraph API.
• Adds common integration tests and documentation for the shared runtime integration contract.

Verification

• python3 -m pytest tests/ -v --tb=short
• python3 -m pytest tests/integrations/common tests/integrations/langgraph --cov=prooflayer.integrations.common --cov=prooflayer.integrations.langgraph --cov-report=term-missing
• python3 -m compileall prooflayer/integrations/common prooflayer/integrations/langgraph prooflayer/evals/adversarial_suite.py

Notes

• Full suite passes on local Python 3.9.6: 351 passed.
• Focused common + LangGraph integration coverage: 93%.
• Existing MCP runtime wrapper tests remain green.
• Phase 2 should start with LangChain MCP and LlamaIndex after confirming exact dependency version floors.