feat: hub import action and an explicit marker for unmanaged sessions

[bthall]

Summary

Two hub improvements.

Import from the hub

i on a running session that isn't in the config runs the same capture as mox import (structure, pane geometry, SSH connections) and returns to a fresh hub with the session now configured. The status line reports imported <name>, plus the first geometry warning when the capture degraded, or the error if the import failed. i on a configured session reports that it's already in the config.

Plumbing note: the import runs between hub visits in runPicker's loop (same pattern as the editor handoff), so the reloaded config and manager always agree. A small hubNote carries the result into the next hub's status line.

Unmanaged sessions are visibly marked

Configured vs tmux-only sessions were only distinguishable by name color (green vs yellow), which is easy to miss. mox list already has an ORIGIN column for this. The hub now uses the same vocabulary: a tmux tag on the list row meta and tmux only in the preview title.

Testing

• New model tests TestHubImportKey and TestHubUnmanagedMarker, written failing first.
• make test and golangci-lint v2 clean. No new tmux syntax: the import path reuses inspectSession and appendSessionToConfig, both already covered.

Docs: hub key table and origin-marker note in docs/commands.md, README feature bullet, changelog under Unreleased.