feat: add Reset Stats button to admin LLM tracking panel

Allows resetting the LLM stats aggregation window without deleting
historical data. Stores a reset timestamp in metadata and filters
the aggregation query to only include calls after that point.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/superpowers/specs/2026-03-13-llm-stats-reset-design.md

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+# LLM Stats Reset Button
+
+## Goal
+
+Add a "Reset Stats" button to the Admin panel's LLM tracking column that gives a clean aggregation window without deleting historical data.
+
+## Behavior
+
+1. "Reset Stats" button appears at the bottom of the right column, below the TOTAL row.
+2. Clicking it emits `admin-reset-llm-stats` socket event.
+3. Server stores the current timestamp in the `metadata` table as `llm_stats_reset_after`.
+4. Server's `getAggregatedStats()` adds `WHERE timestamp > ?` using that value.
+5. Server responds with updated (now-empty) stats via existing `admin-llm-stats-result`.
+6. Client renders the refreshed stats as usual.
+
+## Changes
+
+- **`server/src/llm/llmStatsService.ts`** — `getAggregatedStats()` reads reset timestamp from metadata, filters query. New `resetStats()` method to store the timestamp.
+- **`server/src/network/SocketServer.ts`** — Handle `admin-reset-llm-stats` event: call `resetStats()`, respond with fresh stats.
+- **`client/src/ui/AdminPanel.ts`** — Add "Reset Stats" button in `updateLlmStats()`, wire click to new callback.
+- **`client/src/scenes/GameScene.ts`** — Wire the new callback through to SocketClient.
+- **`client/src/network/SocketClient.ts`** — Add `resetLlmStats()` method emitting the event.