bug: reject duplicate options in release-artifact build

[codeforester]

Goal

Make scripts/release-artifact build reject ambiguous duplicate singleton options.

Background

The build parser accepts repeated --version, --output, and --commit flags and silently keeps the last value. For example, scripts/release-artifact build --version 2.0.0 --version 2.0.0 --output DIR succeeds today, and differing values select the final version without warning. The guarded scripts/release command already rejects duplicate singleton options, so the two release entry points expose inconsistent CLI contracts.

Scope

• Track occurrence of each singleton build option and return usage status 2 on a duplicate, before repository or filesystem side effects.
• Keep missing-value and unknown-option diagnostics deterministic.
• Add focused tests for duplicate equal and differing values, including --commit and --output.
• Document the singleton-option rule in the command usage or release-process documentation.

Acceptance Criteria

• Each of --version, --output, and --commit is accepted at most once.
• Duplicate options fail with a clear diagnostic and do not create or modify an artifact directory.
• Existing valid builds and deterministic verification remain unchanged.

Validation

• ./tests/validate.sh
• Focused tests/release-artifact.bats cases for duplicate options and no-side-effect behavior.

Non-Goals

• Changing release version grammar or publication behavior.
• Adding new release-artifact options.

Project Fields

• Status: Backlog
• Priority: P2
• Area: Packaging
• Initiative: Adoption Polish
• Size: S
• Milestone: v2.1.0

Agent Assignment

• Assignee: codeforester