Fix: Make CI .env manipulation robust against missing trailing newlines (#15189)

## Problem

CI workflow started timing out on October 14, 2025 after commit
`d750df7fff` removed the trailing newline from `.env.example`.

## Root Cause

When `.env.example` lacks a trailing newline:
```bash
# Last line without newline
# CLICKHOUSE_URL=...twenty
```

And CI runs:
```bash
echo "NODE_PORT=3002" >> .env
```

Result:
```bash
# CLICKHOUSE_URL=...twentyNODE_PORT=3002  ← Commented out!
```

Server starts on default port 3000 instead of 3002, health check fails.

## Fix

1. **Restore trailing newline** to `.env.example`
2. **Make all CI `.env` operations robust** by adding `echo "" >> .env`
before appending
3. **Simplified `set_env_var`** function to always add newline first

Now works regardless of whether template files have trailing newlines.

## Files Changed

- 6 CI workflow files
- 1 .env.example file

.github/workflows/ci-cli.yaml

@@ -81,6 +81,7 @@ jobs:
       - name: Server / Append billing config to .env.test
         working-directory: packages/twenty-server
         run: |
+          echo "" >> .env.test
           echo "IS_BILLING_ENABLED=true" >> .env.test
           echo "BILLING_STRIPE_API_KEY=test-api-key" >> .env.test
           echo "BILLING_STRIPE_BASE_PLAN_PRODUCT_ID=test-base-plan-product-id" >> .env.test