fix: correct grammar in RateLimitError messages

Addresses Cursor Bugbot review comment on PR #65.

## Issue
Error messages showed grammatically incorrect output:
- "in 1 minutes" (should be "in 1 minute")
- "in 0 minutes" for past reset times (should be "should be available now")
- Always showed minutes even for short durations under 60 seconds

## Fix
- Handle singular vs plural correctly ("1 second" vs "2 seconds", "1 minute" vs "2 minutes")
- Show seconds for durations under 60 seconds ("in 30 seconds")
- Show "should be available now" when reset time is in the past
- Update tests to expect correct grammar

## Tests
- Updated existing tests to expect correct output
- Added new tests for singular/plural edge cases
- All 9 rate limit tests passing

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

packages/core/src/__tests__/rate-limit.test.ts

@@ -55,7 +55,7 @@ describe("RateLimitError", () => {
     const error = new RateLimitError("rest", resetAt);
 
     expect(error.retryAfter).toBe(30);
-    expect(error.message).toContain("1 minutes"); // Ceiling of 30s / 60 = 1 min
+    expect(error.message).toContain("30 seconds"); // Shows seconds, not minutes
 
     vi.useRealTimers();
   });
@@ -70,6 +70,7 @@ describe("RateLimitError", () => {
 
     // retryAfter should be 0 (already reset)
     expect(error.retryAfter).toBe(0);
+    expect(error.message).toContain("should be available now");
 
     vi.useRealTimers();
   });
@@ -94,4 +95,40 @@ describe("RateLimitError", () => {
     const searchError = new RateLimitError("search", resetAt);
     expect(searchError.resource).toBe("search");
   });
+
+  it("should handle singular forms correctly", () => {
+    vi.useFakeTimers();
+    const now = new Date("2026-02-16T10:00:00Z");
+    vi.setSystemTime(now);
+
+    // 1 second
+    const resetAt1s = new Date("2026-02-16T10:00:01Z");
+    const error1s = new RateLimitError("rest", resetAt1s);
+    expect(error1s.message).toContain("1 second"); // singular, not "1 seconds"
+
+    // 1 minute
+    const resetAt1m = new Date("2026-02-16T10:01:00Z");
+    const error1m = new RateLimitError("rest", resetAt1m);
+    expect(error1m.message).toContain("1 minute"); // singular, not "1 minutes"
+
+    vi.useRealTimers();
+  });
+
+  it("should handle plural forms correctly", () => {
+    vi.useFakeTimers();
+    const now = new Date("2026-02-16T10:00:00Z");
+    vi.setSystemTime(now);
+
+    // 2 seconds
+    const resetAt2s = new Date("2026-02-16T10:00:02Z");
+    const error2s = new RateLimitError("rest", resetAt2s);
+    expect(error2s.message).toContain("2 seconds");
+
+    // 2 minutes
+    const resetAt2m = new Date("2026-02-16T10:02:00Z");
+    const error2m = new RateLimitError("rest", resetAt2m);
+    expect(error2m.message).toContain("2 minutes");
+
+    vi.useRealTimers();
+  });
 });

packages/core/src/types.ts

@@ -53,8 +53,22 @@ export class RateLimitError extends Error {
       hour: "2-digit",
       minute: "2-digit",
     });
+
+    // Format retry time as human-readable string with correct grammar
+    let retryMessage: string;
+    if (retryAfter === 0) {
+      retryMessage = "should be available now";
+    } else if (retryAfter < 60) {
+      const plural = retryAfter === 1 ? "" : "s";
+      retryMessage = `in ${retryAfter} second${plural}`;
+    } else {
+      const minutes = Math.ceil(retryAfter / 60);
+      const plural = minutes === 1 ? "" : "s";
+      retryMessage = `in ${minutes} minute${plural}`;
+    }
+
     super(
-      `Rate limit exceeded for ${resource}. Resets at ${resetTime} (in ${Math.ceil(retryAfter / 60)} minutes)`,
+      `Rate limit exceeded for ${resource}. Resets at ${resetTime} (${retryMessage})`,
       options,
     );
     this.name = "RateLimitError";