ci: schedule weekly run for ci_full.yml

[dtsong]

Problem

ci_full.yml only runs on PRs to master. Upstream Docker image breaks (Presto, Trino, ClickHouse, Vertica) go undetected between PRs.

Fix

Add cron schedule trigger:

on:
  pull_request:
    branches: [master]
  schedule:
    - cron: '0 6 * * 1'  # Monday 6am UTC

Acceptance Criteria

• ci_full.yml runs weekly on schedule
• Failures notify maintainers (GitHub notification or Slack)