feat(git): import admitted source heads

[zhiiw]

Summary

• add the first Gitoxide object data-plane operation: exact admitted source HEAD import
• bind source path/commit/tree to the existing opaque repository-admission capability
• create only a fresh Maka-owned SHA-1 bare repository and deterministic baseline ref
• reject unsupported paths, entries, collisions, stale HEADs, quotas, and existing destinations

Primary invariant

Only the owner of an opaque repository-admission capability can import the exact SHA-1 HEAD captured by that capability. The helper copies only reachable tree/blob objects into a fresh destination and publishes a deterministic zero-parent baseline under refs/maka/*.

Atomicity and recovery

The fresh destination is the artifact boundary. Before the baseline ref is published, copied objects are unreachable implementation residue. Until a complete success response is returned, the caller must treat the entire destination as untrusted and discard it. No system Git fallback is permitted.

Scope

API-only stacked Draft. No T1/T2, Desktop/CLI consumer, projection, candidate, or successor CAS is added here.

Verification

• Runtime Host/core/storage/runtime builds pass locally.
• Focused TypeScript authority tests pass; real-helper cases are exercised by the three-platform Gitoxide workflow.
• Local Rust execution is unavailable on this Windows machine, so Cargo compile/test evidence is delegated to CI.

中文说明

本切片只增加 Gitoxide 的 source import 数据面：只有 repository admission authority 签发的 opaque capability 才能决定 source path、HEAD commit 与 tree。helper 只把该 exact HEAD 的 reachable tree/blob 复制到全新的 Maka-owned SHA-1 bare repository，并发布确定性的零父 baseline ref。

fresh destination 是失败与回滚边界；完整成功响应返回前，上层必须把整个目录视为不可信并整体丢弃。不会 fallback 到系统 Git，也不在本 PR 接入 T1/T2、Desktop/CLI、projection 或 successor CAS。

[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Superseded by apache#3561.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。