Add AGENTS.md with Cursor Cloud development environment instructions

AGENTS.md

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+# AGENTS.md
+
+## Cursor Cloud specific instructions
+
+This repository contains a single `postgresql.conf` configuration file for **PostgreSQL 16**.
+
+### Service overview
+
+| Service        | Details |
+|----------------|---------|
+| PostgreSQL 16  | The only service. Config lives at repo root `postgresql.conf`. |
+
+### Starting PostgreSQL
+
+```bash
+sudo cp /workspace/postgresql.conf /etc/postgresql/16/main/postgresql.conf
+sudo chown postgres:postgres /etc/postgresql/16/main/postgresql.conf
+sudo pg_ctlcluster 16 main start
+```
+
+### Key non-default config values
+
+- `port = 5432`
+- `max_connections = 1000`
+- `shared_buffers = 128MB`
+- `ssl = on` (uses snakeoil certs at `/etc/ssl/certs/ssl-cert-snakeoil.pem`)
+
+### Gotchas
+
+- The `ssl = on` setting requires the `ssl-cert` snakeoil certificates. The `postgresql-16` Ubuntu package automatically adds the `postgres` user to the `ssl-cert` group, so this works out of the box after `apt-get install postgresql-16`.
+- There are no application-level tests, lint, or build steps in this repo — only the PostgreSQL config file. Validation means confirming `SHOW` commands return the expected values after starting PG with this config.
+- Use `sudo -u postgres psql` to connect (peer auth by default via `pg_hba.conf`).