refactor: collapse the no-arg command preamble into parsePlain

[andyrewlee]

What

The 11 read/no-arg commands — abort, bottom, continue, fold, guide, log, repair, status, top, undo, validate — each opened with the identical 8-line preamble:

var asJSON bool
fs := newFlagSet("X", &asJSON)
if err := parseFlagSet(fs, args); err != nil { return err }
if err := rejectArgs("X", fs.Args()); err != nil { return err }

Extracted into parsePlain(command, args) (bool, error) in cmd/util.go. Each call site is now 4 lines, and the "takes no positionals, only --json" contract has a single definition.

Why it's safe

Pure mechanical extraction — no behaviour change. The flag-bearing commands (create, modify, restack, sync, …) are untouched; only the commands whose sole flag is --json and that take no positionals were converted.

make ci green — stdlib-only, lint 0 issues, race + e2e, coverage 86.4%. Net −28 lines.