feat(mutagen): retire a pairing once it holds nothing

[gapul]

#182 stopped directories from being recreated on a schedule, but said nothing about the ones already there: a session lives forever once created, so ~/Sync still accumulated an empty directory per host, which is the clutter the whole rule was meant to avoid.

A pairing is now retired on the next ssh when all of these hold:

• empty on both sides (files 0, symlinks 0, directories ≤ 1 — the root itself counts as 1). Nothing can be lost by construction; anything with content is left alone.
• untouched for an hour, by the local directory's own mtime. A directory never disappears while it is being used, including the case where you pair a host now and drop a file in shortly after.
• not the host being paired in this very run, and rooted under ~/Sync — sessions created by hand elsewhere are not this module's business.

Terminating also removes the local directory and the state marker, so the next ssh to that host pairs it again from scratch.

Checked against stub mutagen with three sessions in different states:

macmini  empty both sides, mtime old     -> retiring empty pair macmini   (dir + marker gone)
rpi4     one file                        -> kept
pve      empty both sides, mtime fresh   -> kept
ispc     the host being ssh'd to         -> created