Split fetch from jobsearch into a separate fetcher command, and renamed fetch once to fts-once

job-search no longer has a fetch command. Its subcommands are now in
cmd/fetcher, which reads the same configuration: "fetcher service" runs
every enabled source's fetcher on its interval until stopped, and
"fetcher fts-once" (was "fetch once") fetches from Find a Tender once and
prints the releases as JSON. The new namea say what the commands do,
because service runs every source while fts-once fetches only from Find a
Tender.

The code stays in internal/cli/fetch, where NewCommands replaces the fetch
parent command. once.go is now fts_once.go and commandOnce is
commandFTSOnce. The README's usage, the config README and the find_a_tender
api_root comment now give the new command names.

cmd/fetcher/main_test.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"slices"
+	"testing"
+)
+
+// The commands are tested in internal/cli/fetch. This test checks newRootCmd
+// puts them together.
+
+func TestCommandTree(t *testing.T) {
+	root := newRootCmd()
+
+	var names []string
+	for _, c := range root.Commands() {
+		names = append(names, c.Name())
+	}
+
+	for _, want := range []string{"fts-once", "service"} {
+		if !slices.Contains(names, want) {
+			t.Errorf("fetcher has no %q command; has %v", want, names)
+		}
+	}
+
+	for _, flag := range []string{"config", "debug", "quiet"} {
+		if root.PersistentFlags().Lookup(flag) == nil {
+			t.Errorf("fetcher has no --%s flag", flag)
+		}
+	}
+}