Add SandBase Harness MCP server

[denial123789]

Summary

Add SandBase Harness to the Code Execution category and provide a structured MCP Index metadata sidecar.

The entry describes the v0.3.1 stdio MCP bridge for persistent sandboxed agent sessions. The metadata enables TensorBlock to generate a searchable profile and install-config preview with:

• immutable source release and documentation
• source-build command and a repository-owned MCP Dockerfile
• stdio transport and optional runtime API-key boundary
• DeepSeek Harness client compatibility
• all six exposed MCP tools
• Apache-2.0 license and self-reported verification evidence

Validation

• npm run typecheck
• npm test — 13 test files, 77 tests passed
• npm run catalog:build — 7,761 entries generated; the expected github-sandbaseai-sandbase-harness-7a5986ca profile resolved with installReady: true
• git diff --check

The v0.3.1 upstream release gate independently passed 572 tests (15 skipped), production builds, package validation, and CLI/workspace smoke coverage. The upstream Dockerfile was subsequently built successfully in GitHub CI as part of sandbaseai/sandbase-harness#35.

Disclosure: this contribution was prepared with AI assistance for the SandBase project.

[denial123789]

Updated the listing and metadata to the immutable v0.3.1 release. Local validation remains green: typecheck, 13 test files / 77 tests, and a 7,761-entry catalog build.

[denial123789]

Closing this Harness entry to avoid multiple SandBase submissions in the same repository. The focused SandBase CLI MCP bridge contribution remains in #1749.