fix: Remove npm latest install, add PR checks, improve error handling and formatting

[glesage]

Summary

This PR cleans up the CI pipelines, fixes formatting, improves error handling with proper error chaining, and updates the TypeScript target to ES2022.

Changes

CI & Tooling

• New PR checks workflow (.github/workflows/pr-checks.yml): Adds automated CI that runs lint, format check, build, example build/test, and unit tests on every PR to catch issues early.
• Remove npm install -g npm@latest (.github/workflows/publish-npm.yml): Eliminates an unnecessary global npm upgrade step from the publish workflow.
• Add format:check script (package.json): Adds a bun run format:check script for CI to verify formatting without writing changes.

Error handling

• Error chaining with .cause across bedrock-client.ts, config.ts, and validators.ts: Wraps caught errors with the original error attached via Error.cause, enabling better debugging by preserving the full error chain.

Bug fixes

• Fix OpenRouter SDK API (openrouter-client.ts): Wraps chat parameters in a chatRequest object to match the updated SDK interface.

Formatting

• Fix self-closing textarea formatting (PromptModal.svelte): Closes textarea tags on the same line to satisfy prettier formatting rules.
• Update TypeScript target (tsconfig.json): Bumps target and lib from ES2020 to ES2022, which is required for native Error.cause support.