fix(hook): make check-messages.sh JSON sanitizer work without python3 on PATH

sanitize_json() called `python3` unconditionally, but on ACG Windows boxes
the Microsoft Store python3 alias is disabled and `py` is the launcher
(feedback_python_windows). When `python3` was missing the function silently
returned empty, and the surrounding `result_safe='{"messages":[]}'` default
dropped every unread coord message — no error, no warning, no toast.

Now prefers identity.json's `.python.command` (set during machine onboarding,
matching the pattern other scripts already use), falls back to
`command -v python3 || command -v py || command -v python`, and if no Python
is available falls back to `tr -d '\000-\037'` so jq can still parse — lossy
on real \n/\t in string fields but keeps messages visible instead of dropping
them.

.claude/scripts/check-messages.sh

@@ -22,19 +22,38 @@ if [ -f "$IDENTITY_FILE" ]; then
   USER_ALIAS=$(jq -r '.user // empty' "$IDENTITY_FILE" 2>/dev/null)
 fi
 
+# Resolve a Python interpreter for the JSON sanitizer. Prefer the value in
+# identity.json (.python.command — set during machine onboarding); fall back
+# to a PATH lookup. On ACG Windows boxes the Microsoft Store `python3` alias
+# is disabled and `py` is the canonical launcher, so an unconditional
+# `python3` call would silently fail and cause every coord message to be
+# dropped. If no Python is available at all, the sanitizer falls back to
+# `tr -d` (lossy on real \n/\t in string fields but keeps messages visible).
+PY=""
+if [ -f "$IDENTITY_FILE" ]; then
+  PY=$(jq -r '.python.command // empty' "$IDENTITY_FILE" 2>/dev/null)
+fi
+if [ -z "$PY" ]; then
+  PY=$(command -v python3 || command -v py || command -v python)
+fi
+
 # Sanitize JSON: the coord API sometimes returns message bodies with unescaped
 # control chars (U+0000-U+001F) — invalid JSON per RFC 8259, which makes jq
 # abort with "Invalid string: control characters ... must be escaped". Round-
 # trip through python's json.loads(strict=False) (which accepts them) and
 # re-emit properly escaped JSON so jq downstream works on every payload.
 sanitize_json() {
-  python3 -c '
+  if [ -n "$PY" ]; then
+    "$PY" -c '
 import json, sys
 try:
     print(json.dumps(json.loads(sys.stdin.read(), strict=False)))
 except Exception:
     pass
 ' 2>/dev/null
+  else
+    tr -d '\000-\037'
+  fi
 }
 
 # --- Unread messages ---------------------------------------------------------