fix(cron): allow clearing an existing workdir on edit

`updateJob` only emitted `--workdir <path>` when the value was non-empty,
so once a workdir was set on a job, the user had no way to remove it
through Scarf — clearing the TextField and saving was a silent no-op.

Hermes' `cron edit --workdir` argparse documents passing an empty string
as the explicit clear gesture (mirroring the existing `--script` shape,
which already passes empty through here). Drop the `!isEmpty` predicate
so a non-nil value — including "" — reaches the CLI.

The previous capability gate keeps this safe on pre-v0.12 hosts: CronView
passes `workdir: nil` there, so the flag is omitted and v0.11 argparse
is never asked about an unknown arg.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scarf/scarf/Features/Cron/ViewModels/CronViewModel.swift

@@ -161,7 +161,10 @@ final class CronViewModel {
             for skill in newSkills where !skill.isEmpty { args += ["--skill", skill] }
         }
         if let script { args += ["--script", script] }
-        if let workdir, !workdir.isEmpty { args += ["--workdir", workdir] }
+        // `nil` = caller didn't touch the field (omit the flag). Empty string
+        // = user cleared an existing workdir; Hermes documents `--workdir ""`
+        // on edit as the explicit clear gesture, mirroring the `--script` shape.
+        if let workdir { args += ["--workdir", workdir] }
         runAndReload(args, success: "Updated")
     }