## Why

The schema package currently duplicates validation logic: Superstruct schemas mirror Drizzle table definitions by hand. This is error-prone (they can drift) and requires maintaining a separate `validation/` and `types/` module. Drizzle provides first-class Valibot integration (`drizzle-orm/valibot`) that derives insert/update/select schemas directly from table definitions, eliminating duplication entirely. Valibot is also tree-shakeable, which benefits the web app's bundle.

## What Changes

- **BREAKING**: Replace Superstruct with Valibot across `@workspace/schema`
- Remove `packages/schema/src/validation/` module (hand-written Superstruct schemas)
- Remove `packages/schema/src/types/` module (manual type re-exports no longer needed)
- Add Valibot-derived schemas via `createInsertSchema` and `createUpdateSchema` from `drizzle-orm/valibot`
- Add `valibot` dependency; remove `superstruct` dependency
- Update main barrel export in `src/index.ts`
- Validation schemas for enum fields (`status`, `priority`) will use `picklist()` refinements derived from the existing constant arrays in `drizzle/enums.ts`

## Capabilities

### New Capabilities

None — this is a modification of existing capabilities.

### Modified Capabilities

- `validation`: Hand-written Superstruct schemas → auto-derived Valibot schemas from Drizzle tables
- `shared-types`: Manually maintained Infer types → inferred automatically from Valibot schemas via `v.InferOutput`

## Impact

- `packages/schema/package.json` — replace `superstruct` with `valibot` in dependencies
- `packages/schema/src/validation/` — entire directory removed
- `packages/schema/src/types/` — entire directory removed
- `packages/schema/src/index.ts` — exports change to drizzle tables + valibot schemas only
- `apps/api` and `apps/web` import from `@workspace/schema` — no import path changes, but consumers may need to adapt to new API if they used Superstruct directly