fix(remote): stabilize upgrade health and invoke updater

[abeljim]

Summary

• rehydrate remote pane health when scanning live session metadata so the sidebar mismatch indicator stays stable
• invoke the nested remote-agent remote-upgrade command from both the sidebar and Ctrl-R picker action
• add regression coverage for live health hydration and both upgrade command builders

Verification

• cargo fmt --all -- --check
• cargo test -p zellij-server live_session_scan_rehydrates_remote_health_sidecars -- --nocapture
• cargo test -p flock-sidebar -p flock-selector
• cargo test remote_agent
• cargo xtask build --release --plugins-only