fix(quick-dev): delete stale .md renders before rebuilding

render.py rebuilds from scratch per the docstring, but
makedirs(exist_ok=True) only overwrites files that still exist in
the source — stale outputs from renamed/deleted source files linger
in _bmad/render/bmad-quick-dev/ forever. Remove every .md in the
render dir before the render loop; keep the dir itself and any
non-.md files.

Part of plan-quick-dev-python-config-hardening.md (F5).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/bmm-skills/4-implementation/bmad-quick-dev/render.py

@@ -131,6 +131,10 @@ def main():
     out_dir = posixpath.join(root, "_bmad", "render", skill_name)
     os.makedirs(out_dir, exist_ok=True)
 
+    for fname in os.listdir(out_dir):
+        if fname.endswith(".md"):
+            os.remove(posixpath.join(out_dir, fname))
+
     count = 0
     for fname in sorted(os.listdir(script_dir)):
         if not fname.endswith(".md") or fname == "SKILL.md":