Add Vercel sandbox provider support

[tnm]

Summary

• add a new Vercel provider with create/get/list/exec/stream/upload/download/destroy support
• wire Vercel into provider selection, CLI provider status output, and Sandbox auto/manual configuration
• add provider capability contracts in sandboxes/base.py used by CLI capability matrix output
• add unit and integration tests for Vercel provider behavior and CLI provider reporting
• update docs and dependency metadata for Vercel support

Testing

• ruff check .
• black --check .
• python3 -m pytest -q tests/test_vercel_provider.py tests/test_cli.py tests/test_integration_vercel.py

[tnm]

Superseded by split PRs: #19 (capabilities) and #20 (Vercel provider). Closing this combined PR to keep review scope clean.