When installing the Gateway daemon via LaunchAgent (macOS) or systemd (Linux),
environment variables defined in config.env.vars were not being included in
the service environment. This caused API keys and other env vars configured
in clawdbot.json5 to be unavailable when the Gateway ran as a service.
The fix adds a configEnvVars parameter to buildGatewayInstallPlan() which
merges config.env.vars into the service environment. Service-specific
variables (CLAWDBOT_*, HOME, PATH) take precedence over config env vars.
Fixes the issue where users had to manually edit the LaunchAgent plist
to add environment variables like GOOGLE_API_KEY.
