feat: add remote sandbox service adapters

[yanpgwang]

Summary

• add E2B, Tencent CubeSandbox, OpenSandbox, and Daytona adapters behind the internal sandbox provider registry without changing the Managed Agents public API
• persist only provider-owned IDs and fixed-length ownership hashes, then reconstruct clients with Attach after worker restarts
• default remote egress to denied, preserve workspaces across idle pause, confine file paths, and make teardown retry-safe
• add offline cross-provider contract coverage plus opt-in live conformance tests
• add a non-overwriting, mode-0600 local development environment file and literal-value loader that works across worktrees

All remote adapters are documented as Preview. OpenSandbox was additionally verified against a real local OpenSandbox server using its Docker runtime; E2B, CubeSandbox, and Daytona live suites remain opt-in because they require cloud credentials or Linux/KVM infrastructure.

Validation

• go test ./...
• go test -race ./...
• go vet ./...
• golangci-lint v2.12.2 (0 issues)
• documentation typecheck and production build
• TestOpenSandboxLiveConformance against a real Docker-backed OpenSandbox server
• Claude review in focused passes; accepted findings were fixed and protocol-dependent findings were checked against official docs/SDK source