fix(frontend): peek and restart take <array-id>[k] and written job IDs

- apiserver peek (parseJobIDParam): <array-id>[k] now resolves to the
  element; it used to drop [k] and look up the array identity itself
  (and panicked in parseArrayName on some inputs). A job number still
  names that job exactly (an element by its own XCE_JOBID); ranges and
  invalid selections are rejected.
- apiserver restart: the decimal-only pre-check (isValidJobId, now
  unused and removed) rejected the written form c-a-ts-seq[k];
  ParseIdentitySelection, already used for the lookup, validates it.
- xcecli jobs restart converts its argument with ToAPIJobID like the
  control commands, so written IDs work and invalid ones fail locally.
- docs: the peek/restart caveat in the Job arrays section.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/frontend/xcecli/cmd/jobs/restart_test.go

@@ -0,0 +1,42 @@
+package jobs
+
+import (
+	"strconv"
+	"strings"
+	"testing"
+
+	"xcecli/common/format"
+)
+
+// restart sends the API server the decimal job key (or <key>[k]); a job ID
+// in its written form c-a-ts-seq used to be sent unconverted and rejected.
+func TestRestartParseCmdArgsConvertsJobID(t *testing.T) {
+	written := format.FormatJobID(int64(testArrayKey))
+	key := strconv.FormatUint(testArrayKey, 10)
+	cases := []struct{ in, want string }{
+		{key, key},
+		{written, key},
+		{written + "[2]", key + "[2]"},
+		{key + "[2]", key + "[2]"},
+	}
+	for _, c := range cases {
+		r := NewJobRestart(nil)
+		if err := r.ParseCmdArgs(nil, []string{c.in}); err != nil {
+			t.Errorf("%q: unexpected error %v", c.in, err)
+			continue
+		}
+		if r.jobID != c.want {
+			t.Errorf("%q: sends %q, want %q", c.in, r.jobID, c.want)
+		}
+	}
+
+	for _, in := range []string{written + "[1-3]", "garbage"} {
+		r := NewJobRestart(nil)
+		err := r.ParseCmdArgs(nil, []string{in})
+		if err == nil {
+			t.Errorf("%q: want an error", in)
+		} else if !strings.Contains(err.Error(), in) {
+			t.Errorf("%q: error %q does not name the selection", in, err)
+		}
+	}
+}