feat: scope sandboxes to sessions

[yanpgwang]

Summary

• introduce a session-scoped sandbox manager that reuses one sandbox across runs in the same session
• release the sandbox when its session is deleted while preserving isolation between sessions
• document lifecycle semantics and the current restart/orphan-cleanup limitations
• add concurrency, reuse, isolation, failure, and service-level lifecycle coverage

Validation

• go test ./...
• go test -race ./...
• go vet ./...
• git diff --check
• go test -race -count=20 ./internal/sandbox ./internal/app