feat: add VSCode devcontainer setup

[gerardoparrajc]

What changes

Adds a VSCode Dev Container setup (.devcontainer/devcontainer.json) to streamline local development for VSCode users.

Why

This configuration standardizes the development environment inside an isolated container with all required toolchains:

• Base image: Node.js 24 LTS (mcr.microsoft.com/devcontainers/javascript-node:24-bookworm).
• Docker-in-Docker feature: Enabled to support local PostgreSQL containers and allow building the facility-runner:dev runner image.
• Package Manager: Automated global installation of pnpm@11.20.0 via npm in postCreateCommand to avoid symlink permission issues.
• Port Forwarding: Automatically exposes ports 3400 (Web), 4400 (API), 3500 (Docs), and 5461 (PostgreSQL).

Verification

Ran local verification checks prior to opening this PR:

• Installed dependencies inside container: sudo npm install -g pnpm@11.20.0 && pnpm install --frozen-lockfile

• Ran repository verification: pnpm verify (all linters, type checks, and test suites passed)

• pnpm verify passes locally

• Behaviour verified beyond the test suite: Rebuilt and reopened the environment using VSCode Dev Containers, verified Node.js 24 and pnpm 11.20.0 setup, and confirmed pnpm install and service startup work inside the container.

• Documentation updated, or no user-facing change: Internal developer tooling change; added .devcontainer/devcontainer.json for repository contributors.