cmd/assent: 48 copy-pasted stderr failure blocks and three arg-parsing styles #161

Severity: Low-medium (maintainability). Review finding D7; deferred by #157 ("ride the next touch"). Re-verified at 1c4e71e.

• 48 copy-pasted Fprintln(stderr, …) failure blocks in non-test cmd/assent: compare.go 24, test.go 9, doctor_forge.go 4, catalogue.go 3, run.go 3, lint.go 2, render.go 2, main.go 1.
• In compare.go, 22 of them carry the byte-identical "assent compare:", err line.
• 10 bare return 2 in cmd/assent/test.go.
• Three arg-parsing styles in one package: flag.NewFlagSet (compare.go, render.go, run.go), manual positional (lint.go), and a hand-rolled scan (test.go:34-46).

Reproduce: grep -c 'Fprintln(stderr' $(ls cmd/assent/*.go | grep -v _test).

Fix: add a failf(stderr, format, args...) int helper, sweep it in mechanically behind the existing exit-code tests, and converge lint/test on flag.NewFlagSet when next touched.