Clean up and update devcontainer.json

[Copilot]

The devcontainer config added in #217 had stale/unnecessary features and no automatic setup step. This PR trims it down to what's actually needed for contributing to this Docusaurus site.

Changes

• Removed docker-in-docker and git features — neither is needed; git is already in the ubuntu base image
• Updated node feature with explicit version: "24" to match engines.node >= 24.0 in package.json
• Retained github-cli for contributor workflows
• Added postCreateCommand: "cd website && npm ci" so the dev environment is ready to use immediately after container creation

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• containers.dev
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node /home/REDACTED/work/_temp/ghcca-node/node/bin/node --enable-source-maps /home/REDACTED/work/_temp/copilot-developer-action-main/dist/index.js (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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