site service: add --config-file support

[itsjxck]

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AI Disclosure

Claude Fable 5.1 was used to make this change

Description

Pass --config-file through to the service as CONFIG_FILE (resolved to an absolute path, since the service can run from a different working directory). When it is set, --id/--secret/--endpoint become optional; any given flag from that set will still override the config file's value due to env vars being applied after loading config gile in newt. Only non-empty values are written into the service environment.

How to test?

Verifies that --config-file is passed through to the service and that --id/--secret/--endpoint are no longer required when it is set. No real Pangolin server needed.

1. Build: make build
2. Create a dummy config with a fake endpoint:

   mkdir -p /tmp/pgtest && cat > /tmp/pgtest/site.json <<'EOF'
   {"endpoint":"https://invalid.example.test","id":"dummy-id","secret":"dummy-secret"}
   EOF

3. Install using only the config file (relative path, to check it's made absolute):

cd /tmp/pgtest && sudo /path/to/bin/pangolin service install site --config-file site.json

4. Check the service picked the file up:

sudo pangolin service logs site -n 20

Expected: repeated Failed to connect ... https://invalid.example.test ... no such host — that endpoint exists only in the config file, so it must have been loaded via CONFIG_FILE.
5. Clean up:

sudo pangolin service uninstall site && rm -rf /tmp/pgtest

[oschwartz10612]

Thanks so much!