docs: Document workflow activation limitation across skills

Add critical limitation notice that workflow activation is not supported
via API or MCP and requires manual activation in n8n UI.

Changes:
- WORKFLOW_GUIDE.md: Added prominent warning in lifecycle section with
  deployment steps for users
- SKILL.md (workflow-patterns): Added inline warning in deployment checklist
- scheduled_tasks.md: Added activation reminder in deployment checklist
- VALIDATION_GUIDE.md: Added to best practices "Do" section

Context: During practical workflow deployment, discovered that the
n8n_update_partial_workflow operation with active:true setting does not
activate workflows. This is a critical limitation that users must understand
to successfully deploy workflows built via MCP.

Locations updated (4 files):
1. skills/n8n-mcp-tools-expert/WORKFLOW_GUIDE.md (primary)
2. skills/n8n-workflow-patterns/SKILL.md (deployment phase)
3. skills/n8n-workflow-patterns/scheduled_tasks.md (deployment checklist)
4. skills/n8n-mcp-tools-expert/VALIDATION_GUIDE.md (best practices)

🤖 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

skills/n8n-workflow-patterns/SKILL.md

@@ -133,7 +133,7 @@ When building ANY workflow, follow this checklist:
 
 ### Deployment Phase
 - [ ] Review workflow settings (execution order, timeout, error handling)
-- [ ] Activate workflow
+- [ ] Activate workflow ⚠️ **Manual activation required in n8n UI** (API/MCP cannot activate)
 - [ ] Monitor first executions
 - [ ] Document workflow purpose and data flow
 

skills/n8n-workflow-patterns/scheduled_tasks.md

@@ -709,6 +709,7 @@ Use `search_templates({query: "schedule"})` to find more!
 - [ ] Document workflow purpose
 - [ ] Set up monitoring
 - [ ] Configure alerts
+- [ ] Activate workflow in n8n UI ⚠️ **Manual activation required** (API/MCP cannot activate)
 - [ ] Test in production (short interval first)
 - [ ] Monitor first few executions