feat: add Sandbox abstraction — core only (1/N)

[agent-of-mkmeral]

Summary

Add the core Sandbox interface that decouples tool logic from where code runs. This is the first PR in a series to incrementally merge the large PR #1968 into the feature/sandbox branch.

Target: feature/sandbox (not main)

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What's Included (Core Abstraction Only)

New: src/strands/sandbox/

File	Lines	Purpose
base.py	418	Sandbox ABC — 6 abstract streaming/file methods + 2 non-streaming convenience wrappers, ExecutionResult, FileInfo, OutputFile, StreamChunk dataclasses
host.py	380	HostSandbox — native Python file I/O + asyncio.create_subprocess_exec (default)
shell_based.py	200	ShellBasedSandbox — abstract class for remote environments (Docker, SSH)
noop.py	73	NoOpSandbox — raises NotImplementedError for all operations
__init__.py	30	Module exports

Modified

File	Change
src/strands/__init__.py	Added exports: Sandbox, HostSandbox, NoOpSandbox, ShellBasedSandbox, ExecutionResult, FileInfo, OutputFile, StreamChunk, StreamType
src/strands/agent/agent.py	Added sandbox parameter to Agent.__init__ — defaults to HostSandbox() via lazy import
AGENTS.md	Added sandbox/ to directory structure

Tests: 151 passing ✅

Suite	Tests	Coverage
test_base.py	55	ABC contract, dataclasses, convenience methods, streaming/non-streaming
test_host.py	50	Command execution, file I/O (bytes), streaming, timeouts
test_adversarial_security.py	27	Path traversal, injection, language validation, edge cases
test_adversarial_shared_sandbox.py	13	Shared sandboxes, concurrency, multi-agent
test_agent_sandbox.py	7	Agent integration (default HostSandbox, explicit sandbox, tool access)
test_noop.py	12	NoOpSandbox raises NotImplementedError for all operations

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Class Hierarchy

Sandbox (ABC — 6 abstract + lifecycle + helpers)
  ├── HostSandbox (concrete — native Python, default)
  ├── ShellBasedSandbox (ABC — shell-based defaults for remote envs)
  └── NoOpSandbox (concrete — raises NotImplementedError)

Usage

from strands import Agent, HostSandbox, NoOpSandbox

agent = Agent(model=model)                              # default HostSandbox
agent = Agent(sandbox=HostSandbox(working_dir="/tmp/ws")) # explicit
agent = Agent(sandbox=NoOpSandbox())                     # disable

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What's NOT Included (Coming in Follow-up PRs)

1. Vended tools — shell, editor, python_repl in src/strands/vended_tools/
2. AgentSkills sandbox integration — sandbox skill loading
3. CodeActPlugin — P2 per design doc

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Part of #1968 series → feature/sandbox branch

cc @mkmeral