dsh-plugin-verified-search: Add boundaries to DeepSeek Harness's current source retrieval

dsh-plugin-verified-search is an MIT-licensed plugin for DeepSeek Harness, designed to constrain an agent's `web_search` capability into a bounded, verifiable workflow. Its core goal is to ensure source scope, retain evidence, guarantee deterministic JSON selection, and maintain visibility of unresolved gaps when handling variable facts, adopting a fail-closed or explicit unresolved strategy, without verifying publisher authenticity or guaranteeing index recency. The stable version v0.1.1 only provides `verified_search`, supporting post-filtering of structured source hostnames and is suitable for single, narrow variable fact queries. Experimental snapshots include `verified_research` and `verified_json`.