Remove validate_workflow_data tooling and references

[zhongkaifu]

Motivation

• Remove the standalone workflow validation CLI and associated validation helpers to simplify the repo and eliminate the validate_workflow_data surface.
• Remove tests that depended on the standalone validator to avoid stale test coverage after the tool's removal.
• Decouple update_workflow.py from the removed module by inlining action registry loading.

Description

• Deleted the validation CLI and helper module validate_workflow.py and its exported logic for grammar/semantic/schema validation.
• Removed tests tests/test_loop_body_validation.py and tests/test_workflow_parser.py that depended on validate_workflow_data.
• Inlined _load_action_registry into update_workflow.py and replaced the dependency on validate_workflow with direct imports of load_actions_from_path and validate_actions from velvetflow.action_registry.
• Updated documentation to remove references to validate_workflow.py and emphasize using update_workflow.py where applicable in README.md, README.en.md, docs/quickstart.md, docs/quickstart.en.md, and docs/troubleshooting.md.

Testing

• No automated tests were executed as part of this change.

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Codex Task