Add generated Flow Definition authoring skill (#6393)

* Add generated Flow Definition authoring skill

Generate a portable skill from the Flow Definition schema so agents can
author valid declarative flows with the same reference CrewAI uses to
validate them. New declarative flow projects now write this skill.

```python
from crewai.flow.flow_definition import FlowDefinition
skill = FlowDefinition.skill(skips=(), examples_format="yaml")
```

* `examples_format` accepts `"yaml"` or `"json"`.
* Supported skips: `conversational`, `non_linear_flows`, `each`, `hitl`, `persistence`, `config`, `expression_action`, `script_action`, `tool_action`

The generated skill includes authoring rules, a routed crew example, and
an API reference extracted from the Flow, action, state, agent, crew,
and task Pydantic schemas.

* Fix declarative flow scaffold without framework import

* Fix skipped expression action guidance

* Fix markdown links in skill

lib/cli/tests/test_create_flow.py

@@ -26,6 +26,9 @@ def test_create_flow_declarative_project_can_run(
     assert pyproject["project"]["requires-python"]
     assert pyproject["project"]["dependencies"]
     assert (project_root / pyproject["tool"]["crewai"]["definition"]).is_file()
+    agents_md = (project_root / "AGENTS.md").read_text(encoding="utf-8")
+    assert "CrewAI Flow declaration" in agents_md
+    assert "schema: crewai.flow/v1" in agents_md
 
     monkeypatch.chdir(project_root)
     result = CliRunner().invoke(crewai, ["run"], env={"UV_RUN_RECURSION_DEPTH": "1"})