Replace the in-flight updatedb and read PRUNEPATHS out of any quoting

The machines this targets are the ones with an updatedb already grinding
through every snapshot, and `systemctl start` on an active unit is a
no-op. updatedb reads /etc/updatedb.conf once at startup, so a run that
began before the rewrite keeps burning CPU on the old config until it
finishes. Restart the service instead: it's Type=oneshot and plocate
builds into a temp db, so nothing is lost by replacing the run.

Quotes are optional in updatedb.conf, so parse the existing paths out of
whatever quoting the file uses and write the setting back in one
canonical form. `PRUNEPATHS=/tmp` previously fell through to the append
branch and got a second PRUNEPATHS line, which drops /tmp from the
pruned set. Comparing whole paths rather than substrings also keeps a
config that already prunes something like /var/lib/machines/.snapshots
from being mistaken for one that prunes /.snapshots.

Prefer $OMARCHY_PATH over the packaged copy when locating the config
script, per docs/migrations.md, so the migration test exercises the
checked-out script rather than whatever release is installed. Skip when
neither exists: omarchy-migrate runs under set -e, so a missing script
would take down every migration queued behind it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/file-layout.md

@@ -248,7 +248,8 @@ the legacy finalization marker from `~/.local/state/omarchy/` into `done/`.
 finalization. It sources:
 
 - `install/config/*.sh` — theme links, lockout limits, lockscreen PAM,
-  powerprofilesctl shebang fix, docker setup, service enablement, firewall.
+  powerprofilesctl shebang fix, docker setup, Snapper retention, locate
+  index tuning, service enablement, firewall.
 - `install/hardware/all.sh` via `omarchy-setup-hardware` — vendor- and
   device-specific kernel modules, udev rules, microcode, wireless regdom,
   ASUS / Framework / Intel / Apple / Lenovo quirks.