fix: skip retries for non-retryable errors via NonRetryable marker interface

[qozle]

Summary

Fixes #1081. retryWithBackoff retried all errors unconditionally, including WorkflowValidationSkipError (thrown on 401 workflow-not-on-default-branch failures), wasting ~35 seconds per occurrence across 3 retry attempts.

Approach

This PR introduces a NonRetryable marker interface rather than requiring callers to pass an explicit shouldRetry predicate (the approach in #1082). Errors self-declare their retryability:

// src/utils/errors.ts (new)
export interface NonRetryable {
  readonly retryable: false;
}

WorkflowValidationSkipError now implements NonRetryable:

export class WorkflowValidationSkipError extends Error implements NonRetryable {
  readonly retryable = false as const;
  // ...
}

retryWithBackoff defaults to skipping retry for any NonRetryable error, so no changes are needed at existing call sites. Future non-retryable errors (e.g. an HttpError for 4xx status codes, tracked in #1156) get the same behavior automatically just by implementing the interface.

The shouldRetry option is also added to RetryOptions for callers that need explicit control.

Additional fixes

• prepare.ts gap: WorkflowValidationSkipError was not caught in prepare.ts, causing the action to setFailed instead of silently skipping. run.ts already handled this correctly — this PR brings prepare.ts in line.
• Spurious log message: The non-retryable check is now guarded inside attempt < maxAttempts, so "aborting retries" is never printed on the final attempt of a normal retryable error.
• Misleading comments in github-file-ops-server.ts: Two call sites had comments claiming non-403 errors would "fail immediately without retry" — they did not. Comments updated to reflect actual behavior. The underlying fix for those call sites is tracked in retryWithBackoff: non-retryable intent in github-file-ops-server.ts is unenforced #1156.

Tests

17 new tests in test/retry.test.ts covering:

• Baseline retry behavior (preserved)
• NonRetryable default (no explicit shouldRetry needed)
• Explicit shouldRetry option
• Edge cases: maxAttempts=0, non-Error thrown values, shouldRetry called-with assertion
• Integration: real WorkflowValidationSkipError is not retried

All 670 existing tests continue to pass.