Configure minimum and maximum runner counts per VM

[celanthe]

As a: DevOps engineer, I need to configure minimum and maximum runner counts per VM configuration

Why: So I can maintain a warm pool of runners

Acceptance Criteria:

• Configuration accepts min_runners and max_runners parameters
• Validation ensures min_runners <= max_runners
• Backward compatible (min_runners=0 preserves current behavior)

Example Tests:

• Configuration with min_runners: 2 and max_runners: 10 for "Mac-M2" group validates successfully
• Configuration with min_runners: 0 and max_runners: 5 works (pure ephemeral mode)
• Configuration with min_runners: 5 and max_runners: 3 fails validation with clear error message
• Multiple VM configurations can have different min/max settings (e.g., Mac-M2 has min=2/max=10, Mac-Intel has min=1/max=5)