Modernize toolchain, fix RPC relay bugs, and add tests/CI

[cleanunicorn]

Tooling & dependencies:

• Bump to TypeScript 6, ESLint 10 (flat config), Prettier 3, Express 5,
  axios 1.17, ws 8.21, @types/node 22; drop Node 16 for Node 22 LTS.
• Enable TypeScript strict mode and replace implicit any with proper
  shared types (JSON-RPC and Nym protocol interfaces).
• Replace .eslintrc/.eslintignore with eslint.config.mjs flat config.
• Add dotenv support and an env-exit.sample.

Bug fixes:

• Preserve the caller's JSON-RPC id end to end so wallets/MetaMask can
  correlate responses (previously overwritten by the exit's counter).
• Never hang a request: the exit now replies with a JSON-RPC error on
  upstream failure, and the entry fails pending requests on timeout and
  when the Nym connection is not ready.
• Robust websocket handling with error events and automatic reconnect.
• Validate incoming requests as JSON-RPC before relaying.

Docker:

• Multi-stage build on node:22-slim that downloads a pinned, prebuilt
  nym-client binary instead of compiling all of Nym from Rust source.
• Idempotent, signal-forwarding startup script.
• Fix Readme docker run tag (spook -> spook-entry).

Quality:

• Add Vitest unit tests for the protocol helpers.
• Add a GitHub Actions CI workflow (lint, format, typecheck, build, test
  on Node 20 and 22).

https://claude.ai/code/session_01VGxurA1zANfZjMjSixDM4h