Resource usage monitoring per runner

[nicktallen]

Problem

Users have no visibility into how much CPU, memory, or disk each runner consumes. This makes it hard to decide when to pause runners or how many to run concurrently.

Proposed Solution

Display per-runner resource usage in the UI:

• CPU usage (%)
• Memory usage (MB/GB)
• Disk usage for runner work directory
• Optional: network I/O

Acceptance Criteria

• Collect CPU and memory stats per runner process
• Display resource usage in menubar dropdown and/or windowed view
• Aggregate total resource usage across all runners
• Optional alerts when total usage exceeds threshold
• CLI: mac-runner status --resources shows resource info

Notes

Listed in README under "Planned Features." Filing to track formally. Pairs well with the windowed app view (#24) where there's more room to display this data.