fix: update Docker image refs from pyramation/postgres:17 to constructiveio/postgres-plus:18

[pyramation]

Summary

Replaces all pyramation/postgres:17 Docker image references with docker.io/constructiveio/postgres-plus:18 across 8 skill/reference files. This standardizes on the official Constructive Docker Hub image which includes PostGIS, pgvector, and other required extensions.

Files changed: pgpm/references/docker.md, pgpm/references/ci-cd.md, pgpm/references/extensions.md, constructive/references/deployment.md, constructive-ai/references/ollama-ci.md, constructive-ai/references/pgvector-sql.md, constructive-ai/references/rag-pipeline.md, constructive-ai/scripts/setup-rag-database.sh

Review & Testing Checklist for Human

• Verify docker.io/constructiveio/postgres-plus:18 is publicly pullable — run docker pull docker.io/constructiveio/postgres-plus:18 to confirm. If private, all CI examples and the RAG setup script will fail.
• Check PostgreSQL version claim — docker.md line 107 still says "includes PostgreSQL 17" but the tag is :18. Confirm whether :18 means PG18 (in which case the description is wrong) or is just a versioned image tag for PG17.
• Mixed version references in deployment.md and extensions.md — these files now mention both postgres-plus:17 (GHCR) and postgres-plus:18 (Docker Hub) side by side. Verify this is intentional and not confusing for readers.

Notes

• Pure documentation/reference changes — no runtime code affected.
• The setup-rag-database.sh script default PGPM_IMAGE env var changed, so anyone relying on the old default will now pull the new image.
• Companion PRs exist for the same Docker image update in constructive-db and constructive-private-skills.

Link to Devin session: https://app.devin.ai/sessions/d0a20ee2e5194f4e8c342c6bc12fea07
Requested by: @pyramation

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