fix: simplify agent state timestamp calculation

[the3mi]

As suggested in PR #2 review by @juliusv:

In getAgentState you first do secondsAgo := time.Since(latest.modTime).Seconds(), and then you convert it back again. Maybe even cleaner to just return the timestamp to begin with?

Changes

Instead of:

1. Calculate secondsAgo from modTime
2. Return secondsAgo
3. In Collect: time.Now().Unix() - secondsAgo (redundant)

Now:

1. Return Unix timestamp directly from getAgentState()
2. Calculate secondsAgo internally only when needed for state logic
3. In Collect: use timestamp directly (no calculation needed)

This is cleaner and avoids the redundant calculation.