Sandbox snapshot and reattachment checkpoint contract #3222

Parent issue

Sub-issue of #3024.

Problem

Several agents and resources servers retain provider handles to live sandboxes, workspaces, PTYs, or containers. Raw handles are process-local, while copying files may omit process, terminal, baseline, and provider state required for equivalent continuation.

Scope

Define provider-level snapshot/clone, stable logical-handle rebinding, retirement, and atomic multi-sandbox restore. Child issues adapt concrete sandbox-backed agents and environments.

Acceptance criteria

• The provider declares exactly what snapshot captures and excludes.
• Logical handles survive process replacement without reusing stale provider handles.
• Tracked and untracked files, required baselines, and supported PTY state restore equivalently.
• Partial clone failure activates no replacement sessions.
• Restored sandboxes are retired exactly once.
• Next tool action and final verifier result match the pre-failure execution where equivalence is claimed.