perf(harvest): fast-exit when all candidates already archived

Redundant runs (all articles deduped by the 7-day registry) burned ~19
turns / $0.6 verifying nothing new. Prompt now instructs: when every
candidate is already in the registry, return immediately with empty
articles + source_exhausted=true + exhaustion_reason=all_seen — which the
result contract already accepts as SUCCESS (exhaustion is not failure),
also dodging the GLM StructuredOutput misreport for this case.

bin/harvest.sh

@@ -114,6 +114,7 @@ claude -p \
 - 不要使用后台 shell、nohup、& 或任何形式的异步执行。
 - 所有文章抓取完毕后再依次生成摘要并写入 Obsidian，全部完成后再返回结果。
 - 如果某篇文章抓取失败，跳过它继续下一篇，不要因此中断整个任务。
+- 列表页候选与 registry(近7天)对照后,若所有候选均已在库(无任何新文章):立即收尾返回结果——articles 留空、succeeded_count=0、source_exhausted=true、exhaustion_reason="all_seen",不要再做单篇核验、摘要或重复确认,这是正常且成功的结果。
 ${RESULT_REQUIREMENTS}" \
   > "$JSON_OUT" 2> "$RAW_OUT" &
 _claude_pid=$!