Add userDismissedAt field to agents schema

docs/database-migrations.md

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+# Database Migrations
+
+This project uses [drizzle-kit](https://orm.drizzle.team/kit-docs/overview) for database schema management and migrations.
+
+## Overview
+
+- **Schema definition:** `src/db/schema.ts` (drizzle-orm table definitions)
+- **Migration output:** `drizzle/` directory (SQL files + meta journal)
+- **Config:** `drizzle.config.ts`
+- **Runtime migrator:** `src/db/ensure-schema.ts` (calls `drizzle-orm/better-sqlite3/migrator`)
+
+## How It Works
+
+On every server startup, `ensureSchema(db)` runs all pending migrations from the `drizzle/` folder. Drizzle tracks applied migrations in a `__drizzle_migrations` table so only new migrations are applied. This is safe to call repeatedly.
+
+## Workflow
+
+### Making schema changes
+
+1. Edit `src/db/schema.ts` with your table/column changes
+2. Generate a migration:
+   ```bash
+   npx drizzle-kit generate
+   ```
+3. Review the generated SQL in `drizzle/NNNN_*.sql`
+4. Commit the migration file along with your schema change
+
+### Applying migrations
+
+Migrations are applied automatically on server startup. No manual step needed.
+
+For tests, the same `ensureSchema()` function is called on in-memory SQLite databases in `src/db/repositories/drizzle/test-helpers.ts`.
+
+### Checking migration status
+
+```bash
+# See what drizzle-kit would generate (dry run)
+npx drizzle-kit generate --dry-run
+
+# Open drizzle studio to inspect the database
+npx drizzle-kit studio
+```
+
+## Rules
+
+- **Never hand-write migration SQL.** Always use `drizzle-kit generate` from the schema.
+- **Never use raw CREATE TABLE statements** for schema initialization. The migration system handles this.
+- **Always commit migration files.** They are the source of truth for database evolution.
+- **Migration files are immutable.** Once committed, never edit them. Make a new migration instead.
+- **Test with `npx vitest run`** after generating migrations to verify they work with in-memory databases.