dsh-session-link: DSH Plugin for Sharing Context Across Sessions

# Introduction When developing with DSH, you often encounter a situation where Session A has just thoroughly analyzed a problem, Session B takes over the relevant work, and needs Session A's complete context. Without specialized tools, the process involves exporting session logs, transferring the files, unzipping them, and manually reading the zstd-compressed JSONL data—moving files between sessions is a non-trivial cost. dsh-session-link condenses this into a single copy-and-paste operation: click to copy the `@session-<uuid>` reference of the current session from the header, paste it into another session, and send it. The model then invokes the `session_read` tool to read back the dialogue content of that session in `user:` / `assistant:` text format.