feat(studio): add session sandbox environments

[evanlowe]

Summary

• add session-level mounting for multiple sandbox environments and workspace-wide selection
• expose environment discovery, manifest inspection, and sandbox command execution tools with manifest-driven routing
• provision AgentKit Sandbox Tools for AIO-based environments across Volcengine and BytePlus
• add Studio UI for environment manifests, session selection, and environment tool-call states

Validation

• npm test (925 passed)
• npm run build
• targeted Python suite (274 passed)
• post-format Python smoke suite (43 passed)
• pre-commit run
• real AgentKit session E2E: list environments, select environment, inspect manifest, execute anr --help in sandbox
• visual QA at 1440x1000 and 390x844 for session rail, workspace/environment picker, duplicate prevention, and responsive layout