fix(sync-skills): guard the pre-existing prune against an empty prefix (#1087)

prune_stale_links_in_target compared "$resolved" == "$canonical_real/"* while
length-checking only $resolved. If $canonical_real were ever empty the pattern
collapses to == "/"* and matches every absolute path.

The failure is precisely inverted, which is what makes it worth fixing rather
than noting: is_mosaic_skill_name already `continue`s for names that ARE current
mosaic skills, so an empty prefix would delete exactly the FOREIGN symlinks in
every target directory and preserve the mosaic ones. On this host that is 4 base
installs, including codex's own .system entry.

Reported by mos-claude as #1087 after I introduced the same guard in the new
legacy-cleanup path in the previous commit and walked past this instance thirty
lines away. Same defect class, same file, one function apart.

$canonical_real is populated by readlink -f after a mkdir -p, so an empty value
requires readlink to fail — unlikely, but the consequence is deleting operator
symlinks across every harness, which is not a risk worth carrying for one test.

packages/mosaic/framework/tools/_scripts/mosaic-sync-skills

@@ -262,7 +262,12 @@ prune_stale_links_in_target() {
     # -m resolves lexical dangling targets too. If resolution fails, ownership
     # is unproven and the link must be preserved.
     resolved="$(readlink -m "$link_path" 2>/dev/null || true)"
-    if [[ -n "$resolved" && "$resolved" == "$canonical_real/"* ]]; then
+    # $canonical_real must be length-checked BEFORE use as a prefix: if it were
+    # ever empty, "$resolved" == "$canonical_real/"* collapses to == "/"* and
+    # matches every absolute path. Combined with the is_mosaic_skill_name skip
+    # above, that inverts the function precisely — it would delete exactly the
+    # FOREIGN symlinks and keep the mosaic ones. (#1087, reported by mos-claude.)
+    if [[ -n "$resolved" && -n "$canonical_real" && "$resolved" == "$canonical_real/"* ]]; then
       rm -f "$link_path"
       echo "[mosaic-skills] Removed stale retired skill link: $link_path"
     fi