fix(gateway): include resolved node path in systemd unit

[sai-samarth]

Summary

• include the directory of the resolved node binary when generating the gateway systemd unit PATH
• keep the existing venv and node_modules/.bin entries unchanged
• add a regression test covering Node installed under ~/.nvm/.../bin

Problem

On Linux, if Node is installed via nvm, hermes gateway install can generate a systemd unit whose PATH omits the actual Node binary directory. When WhatsApp is enabled, the gateway service then fails the WhatsApp bridge requirement check and can enter a restart loop.

Reproduction

This was reproduced on a fresh Ubuntu 24.04 install with upstream main, Node installed only through nvm, and:

WHATSAPP_ENABLED=true
WHATSAPP_MODE=bot

Then:

hermes gateway install
systemctl --user restart hermes-gateway

The generated unit PATH omitted ~/.nvm/.../bin, node was missing under the service environment, and the gateway crash-looped.

Fix

When generating the systemd unit, resolve node from the current install environment and append its parent directory to the service PATH if present.

Test plan

• ./venv/bin/python -m pytest tests/hermes_cli/test_gateway_service.py -q
• verified the clean repro separately on Ubuntu 24.04 before the fix

Closes #1766