glk1220/dsh-session-git

Merges the work done in a forked session back into the session it came from: /merge turns the branch's session log into a digest without a model call (each turn's prompt and reply, files changed, commands run, choices you made, to-dos and the final answer) and delivers it with your next message, or at once with --wake; merging the same branch again brings only the turns added since. Also adds /branches, a right-sidebar branch map with merge state, a reminder above the composer for branches with unmerged turns, and session_branches.