docs: add error handling guidance to uipath-coded-workflows skill

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Summary

Adds skills/uipath-coded-workflows/GUIDANCE-error-handling.md — a comprehensive reference for building robust, self-recovering coded workflows.

Contents:

• Exception hierarchy table — maps each exception type (HttpRequestException, TimeoutException, FileNotFoundException, etc.) to the correct action (retry, fail immediately, re-authenticate)
• Retry with exponential backoff — reusable RetryHelper class + inline loop pattern with IsTransient() predicate
• Three try/catch templates: queue item processing (ApplicationException vs BusinessException distinction), global exception handler with Slack alert, finally block for guaranteed resource cleanup
• Recovery patterns: Checkpoint/Resume for long-running jobs (JSON-based progress file), dead-letter queue for unprocessable items, circuit breaker to stop hammering failing APIs
• Logging best practices — correct log levels, including context in messages, tracking duration, what never to log (secrets, credentials)
• Quick reference table — scenario → pattern mapping for fast lookup

Motivation

Coded workflows that fail on the first exception and lose progress are a common pain point. The existing skill teaches how to write workflows but doesn't cover how to make them resilient. This guidance file fills that gap without modifying the core SKILL.md, following the pattern established by other GUIDANCE-*.md files in the repo.

Test plan

• Verify RetryHelper C# code compiles against .NET Standard 2.0 (UiPath project target)
• Verify queue item status values (QueueItemStatus.Successful, QueueItemStatus.Failed, QueueItemErrorType) match UiPath.System.Activities API
• Verify system.GetQueueItem, system.SetTransactionStatus, system.AddQueueItem match the system service API in uipath-coded-workflows SKILL.md
• Verify the CircuitBreaker and CheckpointManager are plain C# (no UiPath-specific dependencies) so they can be used as Coded Source Files