AI Coding Rules for TypeScript Strict Projects

# Project Rules — TypeScript Strict

## TypeScript configuration
- `strict: true` is set in `tsconfig.json`. This enables `noImplicitAny`,
  `strictNullChecks`, `strictFunctionTypes`, `strictBindCallApply`,
  `strictPropertyInitialization`, and `noImplicitThis`. Do not disable any of these.
- `noUncheckedIndexedAccess: true` is enabled. Array indexing returns `T | undefined`.
  Always check array access results before using them.
- `exactOptionalPropertyTypes: true` is enabled. Do not assign `undefined` to an
  optional property — omit the property instead.

## Hard rules — no exceptions
- NEVER use `any`. Use `unknown` for values of uncertain type and narrow them
  before use. The single exception is third-party library types that are themselves
  typed as `any` — wrap and re-export with a proper type.
- NEVER use `as T` type assertions unless you add a comment explaining why the
  compiler cannot infer the type and why the assertion is safe. Prefer type guards.
- NEVER use `@ts-ignore` or `@ts-expect-error` without a comment on the same line
  explaining the exact reason. If you find yourself using these, refactor instead.
- NEVER use non-null assertion (`!`) on values that could genuinely be null/undefined
  at runtime. Write an explicit check or use optional chaining (`?.`).
- NEVER widen a type unnecessarily. If a function returns `string`, type it as
  `string` — not `string | undefined` just to avoid a null check.

## Type design conventions
- Model domain errors with discriminated unions, not thrown errors or `null`.
  Return `{ ok: true; value: T } | { ok: false; error: string }` from fallible
  operations instead of relying on catch at the call site.
- Prefer `type` aliases for unions and intersections; use `interface` for object
  shapes that may be extended. Be consistent within a file.
- Generic constraints should be as narrow as possible: `T extends string` instead of
  `T extends unknown` when you only use string operations on `T`.
- Export only what callers need. Keep internal implementation types unexported unless
  there is an explicit reason to expose them.

## Narrowing and runtime validation
- All external data (API responses, form input, `JSON.parse` output, URL params) must
  be validated at the boundary with Zod or a type guard before being used as a typed
  value. Never cast external data with `as MyType` without validation.
- Use `satisfies` to validate object literals against a type without widening:
  `const config = { ... } satisfies Config` instead of `const config: Config = { ... }`
  when you need to preserve literal types.
- Exhaustiveness checks in `switch` statements: add a `default: assertNever(x)` arm
  where `assertNever` is `(x: never) => never` — this catches unhandled union members
  at compile time when the union is extended.

## Definition of done
- `tsc --noEmit` passes with zero errors.
- `grep -r 'as any\|: any\|@ts-ignore\|@ts-expect-error' src/` returns zero results
  (or every hit has an approved justification comment).
- ESLint with `@typescript-eslint/no-explicit-any` and `@typescript-eslint/no-unsafe-*`
  rules enabled passes.
- No `!` non-null assertions on values that come from external data or optional fields.