feat(eslint): disallow 'as' and non-null assertions

[charliecreates]

Context / Motivation

Adds lint rules to prevent TypeScript type assertions using `as` and the non-null assertion operator `!`, per Issue #5.

Changes

• Enable `@typescript-eslint/consistent-type-assertions` with `assertionStyle: 'never'` to forbid `as` (and angle-bracket) casts.
• Enable `@typescript-eslint/no-non-null-assertion` to forbid postfix `!`.

These rules are applied to all `.ts` and `.tsx` files.

Verification

`bun run lint` and `bun run typecheck` pass locally.

Closes #5

[charliecreates]

No functional or stylistic problems were identified in the added configuration lines. The new rules align with the stated goal of disallowing direct type assertions and non-null assertions.

Summary of changes

🔧 ESLint Configuration Updates

1. Added @typescript-eslint/consistent-type-assertions with assertionStyle: 'never' to ban both as and angle-bracket type assertions.
2. Added @typescript-eslint/no-non-null-assertion to forbid the ! postfix operator.
3. Included explanatory comments for both new rules.

These changes strengthen type-safety by removing two escape hatches that can hide type errors.