fix(retro): point the prior-retros context query at files /retro actually writes

Fixes #2552's live half. The gbrain context-query glob targeted
~/.gstack/projects/<slug>/retros/*.md — a directory and extension nothing
writes — so prior-retro recall was dead on every brain-aware run. /retro
saves to .context/retros/*.json (repo-local); the query now reads that. The
issue's second defect (quoted-tilde orphan sweep) is already fixed on main —
the preamble sweeps with "$HOME/..." — verified, no change needed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

retro/SKILL.md

@@ -18,7 +18,10 @@ gbrain:
   context_queries:
     - id: prior-retros
       kind: filesystem
-      glob: "~/.gstack/projects/{repo_slug}/retros/*.md"
+      # #2552: /retro writes .context/retros/*.json (repo-local; see the save
+      # step below) — the old ~/.gstack/.../retros/*.md glob matched a
+      # directory and extension nothing ever writes, so this query was dead.
+      glob: ".context/retros/*.json"
       sort: mtime_desc
       limit: 5
       render_as: "## Prior retros for this project"

retro/SKILL.md.tmpl

@@ -23,7 +23,10 @@ gbrain:
   context_queries:
     - id: prior-retros
       kind: filesystem
-      glob: "~/.gstack/projects/{repo_slug}/retros/*.md"
+      # #2552: /retro writes .context/retros/*.json (repo-local; see the save
+      # step below) — the old ~/.gstack/.../retros/*.md glob matched a
+      # directory and extension nothing ever writes, so this query was dead.
+      glob: ".context/retros/*.json"
       sort: mtime_desc
       limit: 5
       render_as: "## Prior retros for this project"