🤖 fix: correct totalTokens to use last entry, not sum

Previous calculation summed all usageHistory entries, but each entry
already contains cumulative prompt tokens (full context at that turn).
This caused massive over-counting in multi-turn conversations.

Example:
- Turn 1: 1,000 tokens
- Turn 2: 2,500 tokens (includes turn 1)
- Turn 3: 4,200 tokens (includes turns 1-2)

Before: 1,000 + 2,500 + 4,200 = 7,700 tokens (183% inflated)
After: 4,200 tokens (correct - just use last entry)

This fix ensures auto-compaction triggers at actual 70% usage instead
of triggering far earlier due to double/triple counting.

Author: ethanndickson

src/browser/stores/WorkspaceStore.ts

@@ -437,17 +437,17 @@ export class WorkspaceStore {
       const model = aggregator.getCurrentModel();
       const usageHistory = cumUsageHistory(messages, model);
 
-      // Calculate total from usage history (now includes historical)
-      const totalTokens = usageHistory.reduce(
-        (sum, u) =>
-          sum +
-          u.input.tokens +
-          u.cached.tokens +
-          u.cacheCreate.tokens +
-          u.output.tokens +
-          u.reasoning.tokens,
-        0
-      );
+      // Use last entry's total (each entry is cumulative, not a delta)
+      // Each usageHistory entry contains the FULL prompt tokens for that turn,
+      // so we only need the most recent value, not a sum
+      const lastEntry = usageHistory[usageHistory.length - 1];
+      const totalTokens = lastEntry
+        ? lastEntry.input.tokens +
+          lastEntry.cached.tokens +
+          lastEntry.cacheCreate.tokens +
+          lastEntry.output.tokens +
+          lastEntry.reasoning.tokens
+        : 0;
 
       return { usageHistory, totalTokens };
     });