ui: field help text + editable program commands

- Add a 'hint' to the shared Field/TextField; annotate the config forms with
  short, accurate descriptions of each field:
  - program: source (working copy castle builds from), behavior (tool/daemon/
    frontend), stack (optional toolchain template), repo/ref (for clone),
    system deps (listed for reference, NOT auto-installed), commands.
  - service: port (listened-on, health-checked, proxied; map via ${port}),
    health path, proxy path vs proxy host, runs.
  - job: schedule (cron → systemd timer), runs.
- ProgramFields: the Commands section is now editable (build/test/lint/
  type-check/run), so a program with no stack can declare its own dev verbs
  by hand — a declared command overrides the stack default. Verified the
  commands/build shape round-trips through the config API.

App builds clean.

app/src/components/detail/JobFields.tsx

@@ -58,8 +58,16 @@ export function JobFields({ job, onSave, onDelete }: Props) {
   return (
     <div className="space-y-4">
       <TextField label="Description" value={description} onChange={setDescription} />
-      <TextField label="Schedule" value={schedule} onChange={setSchedule} width="w-48" mono placeholder="0 2 * * *" />
-      <Field label="Runs">
+      <TextField
+        label="Schedule"
+        value={schedule}
+        onChange={setSchedule}
+        width="w-48"
+        mono
+        placeholder="0 2 * * *"
+        hint="Cron expression — castle generates a systemd timer that runs the job on this schedule."
+      />
+      <Field label="Runs" hint="The console script or command the job runs on each tick, then exits.">
         <span className="text-sm font-mono text-[var(--muted)]">{runnerLabel(runner)} &middot; </span>
         <input
           value={runTarget}