docs: add SandBase Harness to runtime security list

[liyangbing]

Summary

• add SandBase Harness to Sandboxing & Isolation
• describe only documented runtime capabilities: sandboxed tools, credential boundaries, policy controls, persistent sessions, audit/replay, and MCP
• keep the selected deployment/backend qualification explicit

Verification

• git diff --check passes
• source: https://github.com/sandbaseai/sandbase-harness
• source documentation: https://github.com/sandbaseai/sandbase-harness/blob/main/README.md

This is a single-row documentation change.

[liyangbing]

Verification note:

• SandBase Harness source: https://github.com/sandbaseai/sandbase-harness
• Documentation: https://github.com/sandbaseai/sandbase-harness/blob/main/README.md
• The entry is limited to documented runtime capabilities: sandboxed tools, credential boundaries, policy controls, persistent sessions, audit logs, event replay, and an MCP bridge.
• Isolation and security posture depend on the selected deployment/backend; this is not a claim of universal microVM/kernel isolation, a security certification, or maintainer endorsement.

The PR changes one row in the Sandboxing & Isolation table and currently has no automated checks reported.