feat(bench): implement --resume for interrupted benchmark runs

[bug-ops]

Description

When --resume is passed and a partial results.json exists in the output directory, skip already-completed scenarios and continue from where the run stopped.

Part of epic #2827. See spec: .local/specs/zeph-bench/spec.md FR-008.

Scope

• On startup with --resume: read results.json if it exists, extract completed scenario IDs
• Skip scenarios whose IDs appear in the existing results
• Append new results to existing list; update aggregate on completion
• If results.json does not exist, behave as a fresh run
• Warn if the existing results are from a different dataset or model

Acceptance Criteria

• --resume on a partial results file skips completed scenarios
• Final results.json contains all scenarios (original + resumed)
• --resume with no existing file behaves as fresh run
• Warning printed if dataset or model mismatch between existing and current run