Classroom: Codespaces storage quotas and prebuild guidance

[cuihtlauac]

Parent issue: #24

Problem

Free Codespaces accounts have limited storage and compute. Pre-builds consume quota even when idle. Teachers recommend enabling prebuilds only right before a lesson and disabling after. If the container image exceeds the free-tier storage limit, it won't start.

Supporting evidence from UCSD study (Valstar et al., ITiCSE 2020)

Students running DevContainers locally reported three resource-related disadvantages:

• "take a lot of storage"
• "take much memory"
• "run significantly slower"
• Some students' laptops "could not handle running the DevContainer"

This affects both Codespaces (quota limits) and local Docker (underpowered student laptops). OCaml compilation — especially the TSan switch — is resource-intensive, making this particularly relevant.

Proposed changes

• Document the image size (compressed and uncompressed) and verify it fits within GitHub Education tier limits
• Add guidance on Codespaces machine type selection (the hostRequirements in devcontainer.json requests 4 CPUs / 8GB RAM / 32GB storage)
• Document how to configure and manage prebuilds for a GitHub Classroom organization
• Note the monthly Codespaces hours included with GitHub Education (currently 180 core-hours/month for free accounts)
• Document minimum vs recommended hardware specs for local Docker usage
• Provide a Codespaces fallback recommendation for students with underpowered machines