chore: add shared skills catalog (19 skills), installers, manifest, validator

skills/db-migration-safety/SKILL.md

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+---
+name: db-migration-safety
+description: Use when designing database schemas, writing schema migrations, optimizing slow SQL queries, or planning zero-downtime database changes.
+---
+
+# Safe Database Migrations & SQL Optimization
+
+## Purpose
+Ensure all database schema changes execute safely in production without blocking table locks, downtime, or data corruption, while optimizing query execution plans.
+
+## Safe Migration Rules (Zero-Downtime Pattern)
+
+### 1. Adding Columns
+- **Safe:** Adding nullable columns or columns with default values (PostgreSQL >= 11 handles constant defaults instantly without table rewrites).
+- **Unsafe:** Adding `NOT NULL` without a default on large tables (locks table). Use a 3-step migration: add nullable column $\to$ backfill data in batches $\to$ add `NOT NULL` constraint with `NOT VALID` then `VALIDATE CONSTRAINT`.
+
+### 2. Renaming & Dropping Columns
+- **Never rename/drop in one step:** Old application instances running during deployment will crash when the column disappears.
+- **Expand/Contract Pattern:**
+  1. Add new column.
+  2. Write to both old and new columns in application code.
+  3. Backfill existing rows.
+  4. Switch reads to new column.
+  5. Stop writing to old column.
+  6. Drop old column.
+
+### 3. Adding Indexes Safely
+- In PostgreSQL: Always use `CREATE INDEX CONCURRENTLY` (or `DROP INDEX CONCURRENTLY`) to avoid acquiring exclusive write locks.
+- In MySQL: Use `ALGORITHM=INPLACE, LOCK=NONE` or gh-ost / pt-online-schema-change for heavy tables.
+
+## SQL Performance & Query Optimization
+- Analyze queries with `EXPLAIN (ANALYZE, BUFFERS)` (Postgres) or `EXPLAIN ANALYZE` (MySQL).
+- Eliminate Seq Scans / Full Table Scans on high-cardinality tables by adding composite indexes matching query `WHERE` and `ORDER BY` predicates.
+- Prevent N+1 queries by leveraging `JOIN`, eager loading, or DataLoader patterns.