Reattach panes to live nemesis8 containers on launch (session survival, herdr answer)

[kordless]

Goal

When Hyperia launches (or restores a layout per #83), detect still-running n8-* agent containers and offer to reattach panes onto them (n8 resume <session>) instead of spawning fresh shells. The agent already survives Hyperia restarts — nemesis8 runs it detached (-d, no --rm), so the container outlives the terminal. What's missing is only the ergonomics: wiring a pane back onto the surviving agent.

Why

herdr's headline feature is 'detach, agents keep running — reattach from any terminal'. Our architecture is strictly stronger (the agent persists — runtime, filesystem, conversation — not just a PTY session), but reattach is manual today. This closes the last ergonomic gap.

Scope

• On startup (after sidecar up), sweep docker ps for containers named n8-* / image match (the probe already exists in sidecar/src/ghost/api.rs — reuse it)
• Surface a prompt/toast: 'N agent sessions are still running — reattach?'
• Reattach = open a pane running n8 resume <session-id> (n8 already has a resume + picker: nemesis8/src/cli.rs)
• Integration with Restore window layouts on app launch #83: during layout replay, if a saved pane was an n8 session, prefer reattach over fresh shell (store the n8 session id in the saved layout — extends Save window layout on close with modal prompt #82's SavedLayout)

Depends on

Standalone for the boot-sweep + prompt; the layout-replay integration depends on #82/#83.

Context

Follow-up from the herdr (ogulcancelik/herdr) comparison — see the 'Layout Persistence' sticky.