fix: throw HttpError for non-retryable HTTP failures in updateRef

[qozle]

Summary

Fixes #1156. Two updateRef call sites in github-file-ops-server.ts threw plain Error for all non-2xx HTTP responses, so retryWithBackoff retried deterministic failures like 404 (branch not found), 422 (validation failed), and 409 (conflict) — wasting up to 10 seconds per failure.

What changed

Adds HttpError to src/utils/errors.ts:

export class HttpError extends Error implements NonRetryable {
  readonly retryable = false as const;
  constructor(public readonly status: number, message: string) { ... }
}

Both updateRef call sites now throw HttpError for non-403 non-2xx responses. Since HttpError implements NonRetryable, retryWithBackoff skips retries automatically — no shouldRetry option needed at the call sites.

403 intentionally keeps throwing plain Error — the existing comment documents these as intermittent transient GitHub API issues that succeed on retry.

Dependency

This PR builds on #1157 (which introduces NonRetryable and updates retryWithBackoff). It should be merged after #1157. The diff on top of main once #1157 lands is exactly this commit.

Tests

4 new tests in test/retry.test.ts:

• HttpError has retryable=false and isNonRetryable returns true
• HttpError exposes status and name
• HttpError is not retried by retryWithBackoff
• Plain Error (simulating 403) IS still retried

All 674 tests pass.