feat(git): own short-lived helper invocations

[zhiiw]

Summary

This stacked Draft adds one production-shaped invariant above the opaque helper artifact capability in #11:

Runtime Host can launch exactly one short-lived Gitoxide helper only through the designated owner-bound capability, with fixed argv/environment/protocol, bounded I/O, timeout/cancellation, and an exact exit-code/JSON contract.

Exit 0 is accepted only with an exact SHA-1 repository observation; exit 2 only with an unsupported-object-format policy rejection; exit 1 only with an allowlisted stable helper error. Every other exit/response combination fails closed.

Boundary

• Owner: one Runtime Host helper invocation.
• Atomic boundary: artifact revalidation followed by one bounded helper process and one exact response.
• Failure states: invalid request, spawn failure, timeout, abort, output overflow, malformed protocol, stable helper operation failure.
• Rollback: none; repository inspection is read-only.

The three-platform workflow builds the real Rust helper and runs the Runtime Host contract against it. This remains a Draft because the packaged-release signing trust root and Desktop/CLI/managed-workspace consumer are intentionally not part of this slice.

Verification

• Runtime Host TypeScript build
• Real helper invocation contract: SHA-1 success, SHA-256 rejection, unborn SHA-1 failure
• Existing Rust helper protocol: 4 passing tests
• Biome and git diff --check

中文说明

这个 stacked Draft 在 opaque artifact capability 之上只增加一个不变量：Runtime Host 只能通过指定 owner 的 capability 启动一次短生命周期 Gitoxide helper；argv、环境、协议、超时和 I/O 上限全部固定。

exit 0 只接受严格 SHA-1 observation；exit 2 只接受 unsupported object format；exit 1 只接受 allowlist 内的稳定 helper failure。exit code 与 JSON 不一致时统一 fail closed。

三平台 workflow 会编译真实 Rust helper，并由 Runtime Host 验证 SHA-1 成功、SHA-256 拒绝、unborn SHA-1 失败。正式发布签名信任根及 Desktop/CLI/managed-workspace 消费者不在本切片内，因此继续保持 Draft。

[zhiiw]

Superseded by apache#3561. Its implementation and tests were rebuilt from the latest upstream main and included there as part of the consolidated Gitoxide repository admission/import invariant. This extraction PR remains available as design and review history.

中文说明

本 PR 已由 apache#3561 取代。对应实现和测试已从最新 upstream main 重建，并作为完整的 Gitoxide repository admission/import 不变量合入该 Draft。这里继续保留为设计和评审历史。