Add best practices to AGENTS.md: env configuration and testing guidelines

[pyramation]

Summary

Adds a Best Practices section to the root AGENTS.md with two subsections:

1. Environment Configuration — Documents the unified env config system and which package to import depending on where you're working (@pgpmjs/env for PGPM packages, @constructive-io/graphql-env for GraphQL/Constructive packages, pg-env for postgres tools). Includes a good/bad code example.

2. Testing — Documents the four-layer testing framework hierarchy (pgsql-test → graphile-test → @constructive-io/graphql-test → @constructive-io/graphql-server-test), the required beforeEach/afterEach hook pattern, and a list of anti-patterns to avoid (manual pg.Pool creation, using pg-cache in tests, skipping hooks, etc.).

This is a companion to constructive-skills#57 which adds detailed constructive-env and constructive-testing skills.

Review & Testing Checklist for Human

• Verify package import paths are correct — confirm that @pgpmjs/env, @constructive-io/graphql-env, pg-env, pgsql-test, graphile-test, @constructive-io/graphql-test, and @constructive-io/graphql-server-test are the actual importable package names in this monorepo
• Verify the test hooks boilerplate matches the canonical pattern used across existing tests (ordering of beforeEach/afterEach for pg vs db)
• Check that the "which package to import" guidance (pgpm/* → @pgpmjs/env, graphql/* → @constructive-io/graphql-env, postgres/* → pg-env) matches actual usage across the monorepo — particularly whether pg-env is the right recommendation for postgres/* packages

Notes

• Documentation-only change — no code modified, no CI impact expected.
• The anti-patterns listed (manual pg.Pool, pg-cache in tests, manual DB create/drop) were identified from actual instances found in the codebase (e.g., postgres/introspectron/__tests__/introspect.test.ts).

Link to Devin session: https://app.devin.ai/sessions/534e3c7cdabd4b70afea0c91e615ef0b
Requested by: @pyramation

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