fix(mcp): secure HTTP bind and sync tool contract

[dajiaohuang]

Summary

• require a valid signed auth-context secret before the raw D&D Streamable HTTP server may bind to a non-loopback host
• keep stdio, unsigned loopback HTTP, and Hosted 0.0.0.0 plus signed-secret deployments compatible
• align the Skill MCP reference with the runtime's seven CORE_TOOLS, including resolution_presentation
• add regressions that compare the documented core-tool set with runtime policy and cover rejected/allowed non-loopback startup

Security boundary

The Workbench gateway bearer token protects the gateway only. This change prevents an operator from exposing the underlying MCP listener without the signed principal context that Hosted deployments already provide.

Validation

• uv run --no-sync --package sagasmith-dnd-mcp pytest packages/mcp/tests/test_streamable_http_runtime.py packages/mcp/tests/test_skill_contract_reference.py -q
• uv run --no-sync --package sagasmith-dnd-mcp pytest packages/mcp/tests -q
• uv run --no-sync ruff check packages/domain packages/mcp
• uv build --package sagasmith-dnd-mcp --out-dir .artifacts/dist
• git diff --check