fix(retro): keep retro history in the state root, shared across worktrees (#3749)

/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>

docs/skills.md

@@ -1115,7 +1115,7 @@ Claude: Week of Mar 1: 47 commits (3 contributors), 3.2k LOC, 38% tests, 12 PRs,
         [Top 3 team wins, 3 things to improve, 3 habits for next week]
 ```
 
-It saves a JSON snapshot to `.context/retros/` so the next run can show trends.
+It saves a JSON snapshot under `~/.gstack/projects/<slug>/retros/`, shared by every worktree of the repository, so the next run can show trends from any checkout. An older `.context/retros/` is moved there on the first run.
 
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