refactor(ai): deduplicated usage provider coercion helpers

- Deleted usage/shared.ts's duplicate toNumber; every consumer in the directory
  now resolves the single catalog implementation.
- Promoted HOUR_MS, DAY_MS, WEEK_MS, parseIsoTimestamp, parsePositiveTimestamp
  and usageStatus into usage/shared.ts and dropped the local copies.
- Left the epoch coercers, status thresholds and base-url normalizers alone:
  they disagree on the 1e12 boundary, non-positive values and warn levels, so
  merging them would change reported usage.

packages/ai/src/usage/github-copilot.ts

@@ -18,6 +18,7 @@ import type {
 	UsageWindow,
 } from "../usage";
 import { isRecord } from "../utils";
+import { parseIsoTimestamp } from "./shared";
 
 type CopilotQuotaDetail = {
 	entitlement: number;
@@ -74,8 +75,8 @@ function resolveGitHubApiBaseUrl(params: UsageFetchParams): string {
 
 function buildWindow(resetDate: string | undefined): UsageWindow | undefined {
 	if (!resetDate) return undefined;
-	const resetAt = Date.parse(resetDate);
-	if (!Number.isFinite(resetAt)) return undefined;
+	const resetAt = parseIsoTimestamp(resetDate);
+	if (resetAt === undefined) return undefined;
 	return {
 		id: "monthly",
 		label: "Monthly",