Add recommended resource configuration for runner sets

[STRRL]

Background

GitHub-hosted runners provide these default specs:

Runner	CPU	RAM	Storage
ubuntu-latest	4 vCPU	16 GB	14 GB SSD
macos-latest (M1)	3 vCPU	7 GB	14 GB SSD

Currently elastic-fruit-runner has no resource limits — Docker containers and Tart VMs use as much as they can grab. On a constrained host (e.g. 8GB Mac Mini), running multiple runners simultaneously causes memory pressure and heavy swapping.

TODO

• Document recommended max_runners based on host specs (e.g. 8GB → 1 macOS VM + 2 Docker containers)
• Support --cpus and --memory flags for Docker backend
• Support CPU/RAM configuration for Tart VMs
• Add resource config fields to config.yaml (e.g. cpu, memory per runner set)
• Add examples in config.example.yaml