dsh-session-fork: Add Git-like branch management for DSH sessions

dsh-session-fork is an MIT-licensed plugin for DeepSeek Harness (DSH), designed to address the issues of context accumulation and loss in long session management. Drawing inspiration from the Git model, it transforms session forks into manageable branch references and supports the squashing of new rounds between branches and merging them back into the mainline. Core features include intercepting native Fork operations to generate managed branches, supporting the squashing of branch-specific content into the main branch or other branches, and providing a visual branch viewing experience via the built-in VS Code Source Control Graph. Installation requires a web-app-based DSH environment, with the plugin added via a specific command. Typical usage involves first naming the current session the main branch, and for a specified round...