fix: filter out description field in workflow updates (issue #431)

Root Cause:
- n8n recently added a `description` field to workflows
- The field is returned by GET /workflows/:id endpoint
- But PUT /workflows/:id rejects it with "additional properties" error
- Our cleanWorkflowForUpdate() wasn't filtering it out

The Fix:
1. Added `description` to the list of removed fields in cleanWorkflowForUpdate()
2. Added `description` field to Workflow type definition
3. Added test case to verify description field is excluded

Code Review Improvements:
- Removed diagnostic logging code (was only needed for debugging)
- Improved comment accuracy in type definition
- Added comprehensive test coverage for description field

Testing:
- Build succeeds
- New test passes: "should exclude description field for n8n API compatibility"
- All unit tests pass
- Integration tests confirmed fix works with real n8n API

Related: #431

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

src/services/n8n-validation.ts

@@ -140,6 +140,7 @@ export function cleanWorkflowForUpdate(workflow: Workflow): Partial<Workflow> {
     // Remove fields that cause API errors
     pinData,
     tags,
+    description, // Issue #431: n8n returns this field but rejects it in updates
     // Remove additional fields that n8n API doesn't accept
     isArchived,
     usedCredentials,