fix: handle slice(-0) in truncateHeadTail when tail budget is zero

Use floor instead of round and compute right as remainder to guarantee
left + right === limit. Guard slice with right > 0 to avoid slice(-0)
returning the full array.

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

Author: philoserf

src/utils.test.ts

@@ -136,9 +136,10 @@ describe("truncateHeadTail", () => {
   test("handles limit of 1", () => {
     const tokens = ["a", "b", "c", "d", "e"];
     const result = truncateHeadTail(tokens, 1);
-    // 80% of 1 = 1, 20% of 1 = 0 — head portion only
+    // All budget goes to head, tail is empty
     expect(result).toContain("a");
     expect(result).toContain("\n...\n");
+    expect(result).not.toContain("e");
   });
 });
 

src/utils.ts

@@ -88,10 +88,10 @@ export function truncateHeadOnly(tokens: string[], limit: number): string {
 }
 
 export function truncateHeadTail(tokens: string[], limit: number): string {
-  const left = Math.round(limit * 0.8);
-  const right = Math.round(limit * 0.2);
+  const left = Math.max(1, Math.floor(limit * 0.8));
+  const right = Math.max(0, limit - left);
   const leftTokens = tokens.slice(0, left);
-  const rightTokens = tokens.slice(-right);
+  const rightTokens = right > 0 ? tokens.slice(-right) : [];
   return `${joinTokens(leftTokens)}\n...\n${joinTokens(rightTokens)}`;
 }