feat(integrations): add LangChain MCP and LlamaIndex wrappers

[sinewaveai]

What

• Adds prooflayer.integrations.langchain_mcp with SecurityMiddleware.wrap_tools(...) for LangChain-compatible MCP tools.
• Adds prooflayer.integrations.llamaindex with ProofLayerToolWrapper.wrap_tools(...) and retrieved-context scanning.
• Adds shared RuntimeToolSecurity for tool description, argument, output, allowlist/blocklist, audit, and context scanning.
• Adds optional extras for prooflayer-rules[langchain-mcp] and prooflayer-rules[llamaindex].
• Adds docs, runnable local examples, and focused integration tests.

Acceptance coverage

• Tool poisoning is blocked before tool exposure.
• Tool argument exfiltration is blocked before execution.
• Tool output exfiltration is blocked before returning to the agent.
• LlamaIndex retrieved context is scanned and audit events preserve source metadata.
• Existing LangGraph and MCP runtime suites still pass.

Verification

• python3 examples/integrations/langchain_mcp/basic_tool_security.py
• python3 examples/integrations/llamaindex/basic_tool_security.py
• python3 -m compileall prooflayer/integrations/common prooflayer/integrations/langchain_mcp prooflayer/integrations/llamaindex examples/integrations/langchain_mcp examples/integrations/llamaindex
• python3 -m black prooflayer/integrations/common prooflayer/integrations/langchain_mcp prooflayer/integrations/llamaindex tests/integrations/langchain_mcp tests/integrations/llamaindex examples/integrations/langchain_mcp examples/integrations/llamaindex
• python3 -m pytest tests/integrations/common tests/integrations/langchain_mcp tests/integrations/llamaindex --cov=prooflayer.integrations.common --cov=prooflayer.integrations.langchain_mcp --cov=prooflayer.integrations.llamaindex --cov-report=term-missing -> 86% coverage
• python3 -m pytest tests/ -v --tb=short -> 363 passed
• Fresh Python 3.12 venv: pip install -e ".[langchain-mcp,llamaindex]" succeeded
• Fresh Python 3.12 venv import smoke test succeeded for ProofLayer wrappers, langchain_mcp_adapters, and llama_index

Notes

• This PR is stacked on feat(integrations): add shared runtime primitives #10 because Phase 2 depends on the shared primitives introduced there.
• Optional framework imports remain lazy; base package imports do not require LangChain MCP adapters or LlamaIndex.