/retro saved snapshots under .context/retros inside the working tree, so a run
in a linked worktree saw none of the main checkout's history, reported "First
retro recorded", and wrote a snapshot that git worktree remove deletes.
gstack-retro-metrics --history [--next <date>] now resolves the history
directory as <state root>/projects/<slug>/retros (the slug is keyed on the git
common dir, so every worktree shares it), moves any legacy .context/retros from
this worktree and the main checkout into it, lists prior snapshots and prints
the next snapshot path. Step 12/13 and the gbrain prior-retros query use it.
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>
