api.py: save_document — distinguish PATH miss from missing install in error

The systemd unit pins PATH to the venv only, so subprocess.run(['pandoc', ...])
raised FileNotFoundError even though pandoc was installed at /usr/bin/pandoc.
The handler's "pandoc not installed" message was misleading — pandoc was
reachable from a login shell but not from the service. Rephrased to point at
the actual cause: the service's PATH. The systemd drop-in to extend PATH is
not committed here (lives at /etc/systemd/system/aaronai.service.d/path.conf
on the host).

scripts/api.py

@@ -537,7 +537,8 @@ def _execute_save_document(tool_input):
     except subprocess.TimeoutExpired:
         return "save_document: pandoc timed out (>120s)."
     except FileNotFoundError:
-        return "save_document: pandoc not installed."
+        return ("save_document: pandoc binary not reachable from the api process "
+                "(check that PATH in aaronai.service includes /usr/bin).")
     if proc.returncode != 0:
         err = proc.stderr.decode("utf-8", errors="replace")[:400]
         return f"save_document: pandoc failed: {err}"