GH-2: Add AGENTS.md for AI coding agents

AGENTS.md

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+# digib/postgres
+
+A PostgreSQL 18 image with PostGIS 3 and pgvector, published as `ghcr.io/digib/postgres`. Projects use the same image locally, in CI and deployed. Three files: `Dockerfile`, `.github/workflows/publish.yml` (builds for x86 and Arm, checks the extensions, publishes) and `README.md`.
+
+## Public repository
+
+This repository is public. Issues, pull requests, commits and code never name or link the private projects that use the image, or their issues: describe a need in general terms ("a project needs index suggestions").
+
+## Adding an extension
+
+1. Add its package from the PostgreSQL project's apt repository (`postgresql-18-<name>`) to the `Dockerfile`.
+2. Add `CREATE EXTENSION <name>;` to the check in `publish.yml`, with a query that shows it works.
+3. List it in the README's "What's in it".
+
+Every project and deployment using the image gets it, so add only what's needed.
+
+## Checking locally
+
+```bash
+docker build -t postgres:check .
+docker run -d --name check -e POSTGRES_PASSWORD=check postgres:check
+docker exec check psql -U postgres -v ON_ERROR_STOP=1 -c "CREATE EXTENSION postgis; …"   # the workflow's check
+docker rm -f check
+```
+
+A pull request runs the same build and check without publishing.
+
+## Tags
+
+`18` is the latest build, `18-YYYYMMDD` one build, unchanged. Builds run on every push to `main` and every Monday. A new PostgreSQL major version is a new tag, never an update of `18`.
+
+## Workflow
+
+- Every change has an issue. Branch from `main` as `GH-<issue>-short_description`.
+- Commits: `type(GH-<issue>): Description` (Conventional Commits; the description starts with a capital letter).
+- Pull requests: titled `GH-<issue>: Description`, with `Closes #<issue>` and how it was tested.
+- Commit, push, open pull requests and create issues only when asked.

CLAUDE.md

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+@AGENTS.md