chore: update default Docker image to constructiveio/postgres-plus:18

[pyramation]

chore: update default Docker image to postgres-plus:18

Summary

Replaces all references to pyramation/postgres:17 with the new postgres-plus:18 image across CI, the pgpm CLI, and documentation.

Two image variants are used intentionally:

• CI workflow: ghcr.io/constructive-io/docker/postgres-plus:18 (private GHCR, for GitHub Actions)
• pgpm CLI / docs: constructiveio/postgres-plus:18 (public Docker Hub, for local development)

The postgres-plus:18 image ships Postgres 18 with PostGIS, pgvector, and pg_textsearch pre-installed.

Files changed (3):

• .github/workflows/run-tests.yaml — CI service container image
• pgpm/cli/src/commands/docker.ts — default image in help text and runtime default
• graphile/graphile-pg-textsearch-plugin/README.md — testing docs

Review & Testing Checklist for Human

• Verify both images exist and are accessible: Confirm ghcr.io/constructive-io/docker/postgres-plus:18 is pullable from GitHub Actions runners, and constructiveio/postgres-plus:18 is pullable publicly from Docker Hub. If either is missing or misconfigured, all CI or local dev will break.
• Postgres 17 → 18 behavioral changes: This upgrades the Postgres version for all 40+ CI test jobs. Spot-check that no tests depend on Postgres 17-specific behavior. CI passing is a good signal but not conclusive for edge cases.
• Verify GHCR vs Docker Hub images are equivalent: The two registries should serve the same image contents. Confirm they're built from the same Dockerfile/source.

Notes

• A historical reference to pyramation/postgres:17 remains in pgpm/cli/CHANGELOG.md — this is intentionally left as-is since it's a historical changelog entry.
• This PR is intentionally scoped to just the image swap. Matrix changes (adding graphile-settings, graphile-connection-filter to CI) are handled in separate PRs (feat: add integration test suite for ConstructivePreset #807, feat: add dedicated test suite for graphile-connection-filter #806).

Link to Devin Session: https://app.devin.ai/sessions/cf88f3fd383b4421a5169ed01612899d
Requested by: @pyramation

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