fix(JVNAUTOSCI-1426): translate internal GitHub proxy tool names

[witbrock]

Summary

• translate Von internal \github_*\ tool names to the external \@modelcontextprotocol/server-github\ tool names before invoking the proxy
• preserve the internal tool name in responses while surfacing the translated external \proxy_tool\ for diagnostics
• route supported \github_pull_request_read\ modes to the matching external read tool and reject unsupported \github_issue_read\ modes clearly

Verification

• \pdm run pytest tests/backend/test_internal_mcp_github_tools.py -q\
• \pdm run pyright src/backend/integrations/internal_mcp/catalogue.py tests/backend/test_internal_mcp_github_tools.py\
• \pdm run pytest tests/backend/test_workflow_introspection_maintenance_workflow.py -q\
• \pdm run pytest tests/backend/test_mcp_proxy_base_transport_recovery.py -q\
• live smoke via \InternalMCPGateway.invoke('github_get_file_contents', ...)\ against \Strong-AI-Lab/Von\ succeeded after the fix

Notes

• \pdm run pytest tests -m lane_backend_mcp -q\ exceeded the available command timeout twice, so verification used the directly impacted suites plus a live gateway smoke instead.