feat: add schema-constraints reference for safe migration patterns (#30)

* feat: add schema-constraints reference for safe migration patterns

Documents correct approaches for adding constraints in PostgreSQL migrations,
since ADD CONSTRAINT IF NOT EXISTS is not supported and causes syntax errors.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

* Remove additional correct options

---------

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

skills/supabase-postgres-best-practices/AGENTS.md

@@ -74,6 +74,7 @@ Reference files are named `{prefix}-{topic}.md` (e.g., `query-missing-indexes.md
 - `references/query-partial-indexes.md`
 
 **Schema Design** (`schema-`):
+- `references/schema-constraints.md`
 - `references/schema-data-types.md`
 - `references/schema-foreign-key-indexes.md`
 - `references/schema-lowercase-identifiers.md`
@@ -87,4 +88,4 @@ Reference files are named `{prefix}-{topic}.md` (e.g., `query-missing-indexes.md
 
 ---
 
-*30 reference files across 8 categories*
+*31 reference files across 8 categories*

skills/supabase-postgres-best-practices/references/schema-constraints.md

@@ -0,0 +1,80 @@
+---
+title: Add Constraints Safely in Migrations
+impact: HIGH
+impactDescription: Prevents migration failures and enables idempotent schema changes
+tags: constraints, migrations, schema, alter-table
+---
+
+## Add Constraints Safely in Migrations
+
+PostgreSQL does not support `ADD CONSTRAINT IF NOT EXISTS`. Migrations using this syntax will fail.
+
+**Incorrect (causes syntax error):**
+
+```sql
+-- ERROR: syntax error at or near "not" (SQLSTATE 42601)
+alter table public.profiles
+add constraint if not exists profiles_birthchart_id_unique unique (birthchart_id);
+```
+
+**Correct (idempotent constraint creation):**
+
+```sql
+-- Use DO block to check before adding
+do $$
+begin
+  if not exists (
+    select 1 from pg_constraint
+    where conname = 'profiles_birthchart_id_unique'
+    and conrelid = 'public.profiles'::regclass
+  ) then
+    alter table public.profiles
+    add constraint profiles_birthchart_id_unique unique (birthchart_id);
+  end if;
+end $$;
+```
+
+For all constraint types:
+
+```sql
+-- Check constraints
+do $$
+begin
+  if not exists (
+    select 1 from pg_constraint
+    where conname = 'check_age_positive'
+  ) then
+    alter table users add constraint check_age_positive check (age > 0);
+  end if;
+end $$;
+
+-- Foreign keys
+do $$
+begin
+  if not exists (
+    select 1 from pg_constraint
+    where conname = 'profiles_birthchart_id_fkey'
+  ) then
+    alter table profiles
+    add constraint profiles_birthchart_id_fkey
+    foreign key (birthchart_id) references birthcharts(id);
+  end if;
+end $$;
+```
+
+Check if constraint exists:
+
+```sql
+-- Query to check constraint existence
+select conname, contype, pg_get_constraintdef(oid)
+from pg_constraint
+where conrelid = 'public.profiles'::regclass;
+
+-- contype values:
+-- 'p' = PRIMARY KEY
+-- 'f' = FOREIGN KEY
+-- 'u' = UNIQUE
+-- 'c' = CHECK
+```
+
+Reference: [Constraints](https://www.postgresql.org/docs/current/ddl-constraints.html)