ci: restore Vertica to CI or formally deprecate

[dtsong]

Problem

Vertica was dropped from CI in PR #15 with no documentation. The driver (vertica.py, 158 lines) is Tier 2 production-ready, but its support status is ambiguous.

Options

1. Restore to ci_full.yml using the docker-compose Vertica service
2. Formally deprecate with a notice in README and driver code
3. Mark as "community-maintained" in the support matrix

Acceptance Criteria

• Clear decision on Vertica support status
• CI or documentation reflects the decision

[github-actions]

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