fix: retire persistent pane hosts when replacing live panes

[jasonsuhari]

Problem

Some live-pane replacement paths overwrite a PtyPane directly. With keep-running enabled, dropping the old pane disconnects while leaving its host alive, but the replacement snapshot no longer references that host.

Acceptance checks

• Every replacement path explicitly retires the displaced pane host.
• Failed replacement creation leaves the original pane reachable.
• Auth-profile changes do not increase unreferenced pane-host listeners.
• Regression tests exercise keep-running replacements.