bug(cli): `ao review submit` and `ao preview` exit 1 (not 2) on arg-count misuse

[abhay-codes07]

Summary

AGENTS.md states the convention: "Return usage errors as usageError so CLI misuse exits 2; runtime/daemon failures should exit 1." Two commands don't follow it for argument-count misuse — they exit 1 instead of 2.

Where

ao review submit and ao preview declare their positional-arg validators with a bare Cobra validator:

• backend/internal/cli/review.go — Args: cobra.MaximumNArgs(1)
• backend/internal/cli/preview.go — Args: cobra.MaximumNArgs(1) (and preview clear → Args: cobra.NoArgs)

Cobra's arg-count error is a plain errors.New(...). The root's SetFlagErrorFunc only wraps flag-parse errors as usageError, not Args validation, so a too-many-args error is not a usageError and ExitCode returns 1.

Every other command wraps its validator so arg-count misuse becomes a usageError (exit 2): noArgs (root.go), oneSessionIDArg / the claim-pr and rename validators (session.go), and the inline wrappers in project.go and completion.go.

Repro

$ ao review submit mer-1 mer-2
Error: accepts at most 1 arg(s), received 2
$ echo $?
1        # expected 2

Same for ao preview a b and ao preview clear extra.

This is inconsistent with the commands' own tests, which already assert exit 2 for their other usage errors (TestReviewSubmitMissing*IsUsageError, TestPreview_*IsUsageError). A caller that distinguishes usage misuse (2) from a daemon/runtime failure (1) misclassifies these.

Suggested fix

Wrap the arg validators so arg-count misuse returns usageError (exit 2), matching the sibling commands, and reuse the existing noArgs helper for preview clear.