feat: add logging for debug workers availability and prune output

[adityamaru]

Summary

This PR adds logging (without metrics emission) to help understand:

1. How long it takes for debug workers to become available after starting buildkitd
2. The output from the buildctl prune command to understand what's being cleaned

Changes

• Log debug workers availability time when they become ready (e.g., "Debug workers became available after 2500ms")
• Log the full output from buildctl prune command for analysis

Purpose

This is a preparatory step before converting these logs into proper metrics. We want to:

• Verify the debug workers timing information is accurate
• Understand the format of the prune output to properly parse it for metrics

Testing

• The logging is added to existing functions without changing their behavior
• No metrics are emitted yet - just logging for analysis
• Tests continue to pass as no functional changes were made

Next Steps

Once we verify these logs work correctly in production:

1. Parse the prune output to extract bytes reclaimed
2. Convert the logs to proper metrics using the V2 metric types added in FastActions/fa#2201

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