feat: complete Epic 5 story validation and fixes

- Validate and fix Stories 5-3 through 5-8 (CLI and SDK)
- Fix API integration errors (removed non-existent endpoints)
- Align data structures with Story 1.9 and 3.1 definitions
- Fix field naming: workflow_id -> id, Jobs/Steps as arrays
- Add Developer Context sections to all stories
- Remove validation reports for Epic 4 stories
- Update sprint status for Epic 5 stories

Quality improvements:
- Story 5-3: 7.5 -> 9.0 (submit command)
- Story 5-4: 6.5 -> 9.0 (status command)
- Story 5-5: 6.5 -> 9.0 (logs command)
- Story 5-6: 4.5 -> 8.5 (node list command)
- Story 5-7: 7.5 -> 9.0 (Go SDK client)
- Story 5-8: 6.5 -> 9.0 (Go SDK documentation)

All Epic 5 stories now ready for development.

Author: zhaojing1987

README.md

@@ -102,9 +102,16 @@ For detailed deployment instructions, see [Quick Start Guide](docs/quick-start.m
 
 ### 🔌 Extensible Node System
 - **10 Built-in Nodes** - Control flow (condition/loop/sleep) + Operations (shell/http/file) + Docker management
-- **Custom Nodes** - Simple interface for quick business logic extension
+- **Custom Nodes** - Simple interface for quick business logic extension ([Greeter Example](examples/plugins/greeter/))
 - **Plugin-Based** - Node registry with hot-swap support
 
+**Quick Start with Custom Node**:
+```bash
+cd examples/plugins/greeter
+make build test
+# Coverage: 93.8%, Plugin Size: 5.2MB
+```
+
 ### 📊 Enterprise-Grade Observability
 - **Event Sourcing** - Complete event history, all operations traceable
 - **Real-time Log Streaming** - Support `tail -f` mode for viewing execution logs