feat(runtime-host): compose Gitoxide managed sessions

[zhiiw]

Summary

Composes the packaged Gitoxide authority, managed session owner, and hosted tool profile into the Runtime Host product path.

Primary invariant: managed coding is exposed only when the exact packaged helper and session authorities are available before T1.

Owner: Runtime Host execution composition.
Atomic boundary: hosted profile construction.
Failure state: incomplete authority composition reports the profile unavailable.
Rollback: no managed tool is admitted and no fallback mode is selected.

中文说明

本切片把 packaged Gitoxide authority、managed session owner 和 hosted tool profile 接入 Runtime Host 产品路径。只有全部 authority 在 T1 前成立时才开放 managed coding；缺任一能力都明确 unavailable，不静默选择其他模式。

[zhiiw]

Superseded by the rebuilt delivery stack #40 -> #41 -> #42. This extraction PR remains available as implementation and review history; no validated logic was discarded.

中文说明

该 extraction PR 已由从最新 main 重建的组合交付栈 #40 → #41 → #42 替代。这里仍保留实现与审查历史，已验证逻辑没有被丢弃。