feat(runtime-host): own Gitoxide managed sessions

[zhiiw]

Summary

Adds the session-scoped owner that binds managed execution to one admitted Gitoxide workspace and accepted head.

Primary invariant: a session cannot silently switch repository, workspace instance, or accepted head after admission.

Owner: Runtime Host managed session owner.
Atomic boundary: session admission capability issuance.
Failure state: identity drift or unavailable authority fails closed.
Rollback: close revokes the in-memory capability while durable facts remain authoritative.

中文说明

本切片增加 session-scoped managed owner，把执行绑定到唯一的 Gitoxide workspace 与 accepted head。入场后不能静默切换仓库、workspace instance 或 accepted head；身份漂移一律 fail closed。

[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Superseded by the rebuilt delivery stack #40 -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。