dsh-session-sidebar: Add an inbox-style activity view to the DSH sidebar

dsh-session-sidebar (v0.1.0, MIT, maintained by QuentinCrane) adds an inbox-style activity view and a cross-workspace pinned section to the session list of DeepSeek Harness. The activity view aggregates running, pending approval, and unread sessions into a priority area, enabling them to be marked as read with a single click. Three pin modes allow switching between an independent pinned area, workspace-internal pinning, and mixed selection across sessions, supporting workspace tree grouping, drag-and-drop sorting, and full-text session search. It also provides the activity_pins Agent tool for reading and writing the same state. Compatible with DSH Web and Desktop (both with independent profiles), it requires separate installation using `dsh plugin --profile web/desktop`.