Fix tech debt around duration config with duration.Duration types

[devin-ai-integration]

Fix tech debt around duration config

Resolves #35

This PR addresses the tech debt around duration configuration by:

1. Creating a new duration.Duration type to replace integer seconds in config
2. Using util.TimeRange consistently where appropriate
3. Updating code to use the new duration types

Changes:

• Added a new pkg/util/duration package with Duration and RandomDuration types
• Replaced integer seconds fields in config structs with duration.Duration types
• Updated validation functions for all new duration fields
• Updated code to use the new duration types with .ToDuration() method
• Ensured proper use of util.TimeRange for randomized durations

Link to Devin run: https://app.devin.ai/sessions/c7aa6d5f0dc844ceb6f99ff69c62cfa7
User: [email protected]

[devin-ai-integration]

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