refactor(gthulhu): copy the applied set with maps.Clone and tidy comments

[thc1006]

What does this PR change?

Follow-up to #17, no behavior change. In GetChangedStrategies the applied set is rebuilt at the end of each drain. It used a manual make plus a copy loop, now it is maps.Clone(g.strategyMap) which does the same thing. The concurrency test uses for range instead of for i := 0; i < n; i++, and I shortened the comment on the strategyMap / appliedStrategyMap fields so it is not longer than the two fields it describes.

Why is this change needed?

Same reason as the api side follow-up: after #17 merged I read my own code again against modern Go and the comment style this repo prefers. The module targets Go 1.22, so maps.Clone is available and reads better than the hand written copy. The coalescing logic is untouched.

Follow-up to #17.

How was it tested?

On Linux:

• gofmt -s -l on the two files is clean
• go vet ./plugin/gthulhu/... passes
• go test -race ./plugin/gthulhu/... passes, so the concurrency test still guards the write lock under the race detector

Contributor checklist

• The change is focused and does not include unrelated cleanup.
• Tests were added or updated when behavior changed. (no behavior change, existing tests kept)
• Documentation was updated when user-facing behavior changed. (N/A)
• Relevant local/CI checks pass.
• No secrets or sensitive environment data are included.

Scheduling / eBPF changes

N/A. User space plugin code only.