v0.9.18 local runtime transcript hotfix

[spytensor]

Problem

Local v0.9.18 runs can surface three runtime/transcript regressions:

• A persisted Codex resume thread id can be stale, causing raw Session not found for thread_id: ... text to render as an @host reply.
• @user scrollback lines are styled almost like normal body text, making user messages hard to distinguish.
• Root host turns render both the root handoff banner and the host reply header, visually double-labeling @host.

Expected

• Stale Codex resume ids recover by retrying fresh once.
• User-authored transcript lines have a visible identity color.
• Root host turns do not render a duplicate handoff banner; real cross-role handoffs still do.

Validation target

Prepare a v0.9.19 hotfix with tests, clippy, release build, and v0.9 dogfood evidence.