test: add dedupe conflicts error contracts

[CoderDeltaLAN]

Summary

• adds CLI error-contract coverage for dedupe and conflicts
• covers invalid repository roots and symlinked instruction-file failures
• test-only change; no product behavior changed

Checks

• python -m unittest tests.test_cli tests.test_dedupe tests.test_conflicts tests.test_golden_outputs
• ./scripts/check.sh
• ./scripts/post-release-audit.sh

Boundaries

No code, workflow, release, tag, PyPI, dependency, or branch-protection changes.