dsh-content-search: Add keyword search and positioning to the current conversation

dsh-content-search (maintainer: appthin, category: memory, directory page rating: 2 stars) provides keyword search and positioning for the current conversation in DeepSeek Harness. The button is located at the top of the conversation area, and the search box appears in the right sidebar. Matched text is highlighted in yellow, and clicking a result jumps to it. It supports two scopes: "Loaded Content" and "Full History," and is read-only without modifying the conversation. It is suitable for reviewing long conversations. The plugin runs with the permissions of the current process; the source code and license should be checked before installation.