[Repo Assist] perf: replace List.Insert(0) with LinkedList.AddFirst in NotificationHistoryService

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Summary

NotificationHistoryService.AddNotification used List(T).Insert(0, ...) to prepend items, which is O(n) — the entire backing array must shift one position for every insertion. This is the same pattern fixed in ActivityStreamService in commit bd0a7b0 (PR #137).

Root Cause

// Before: O(n) per call
_history.Insert(0, new NotificationHistoryItem { ... });
while (_history.Count > MaxHistory)
    _history.RemoveAt(_history.Count - 1);

Both Insert(0, ...) and (for a full list) the trim loop together cost O(n) per notification. With a MaxHistory of 100, this is minor but unnecessary — and the pattern is easy to fix uniformly across the codebase.

Fix

Replace List(NotificationHistoryItem) with LinkedList(NotificationHistoryItem):

// After: O(1) per call
_history.AddFirst(new NotificationHistoryItem { ... });
while (_history.Count > MaxHistory)
    _history.RemoveLast();

AddFirst and RemoveLast are both O(1) on LinkedList(T). The GetHistory() snapshot via .ToList() (called on read, not write) is unchanged in complexity.

Trade-offs

• LinkedList(T) has higher per-node overhead than List(T) (each node is an object). For a 100-item cap this is negligible (~6.4 KB total vs ~3.2 KB for a List).
• GetHistory() still returns a List(T) snapshot, so callers are unaffected.

Test Status

Suite	Result
OpenClaw.Shared.Tests	✅ All passed
OpenClaw.Tray.Tests	✅ All passed

No production logic was changed — only the collection type and its operation methods.

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