feat: curated failure dataset from traces (`agent-strace dataset`)

[Siddhant-K-code]

Implemented in #78 (v0.37.0). All acceptance criteria met:

• eval dataset add, list, show, export commands
• eval dataset auto with 6 signal filters: has-errors, high-retry, cost-above:N, wide-blast, long-duration:Ns, low-eval-score:N
• Deduplication, --since-days, --label support
• Stored as JSONL in .agent-traces/datasets/
• Zero new dependencies