Require explicit CrewAI project definitions (#6358)

* Require explicit CrewAI project definitions

JSON crews and declarative flows now resolve from `[tool.crewai]`
metadata instead of implicit filename discovery. This makes project type
selection deterministic, prevents stray `crew.json(c)` files from changing
CLI behavior, and centralizes definition path validation for run, install,
deploy validation, plotting, and memory reset paths.

`[tool.crewai].definition` must be a project-local file path. Absolute
paths, `~`, missing files, directories, and paths escaping the project root
are rejected so deploy and runtime commands use the same contract.

Breaking changes and migration paths:

* JSON crew projects are no longer discovered from `crew.json` or
  `crew.jsonc` alone. Add explicit metadata:

  ```toml
  [tool.crewai]
  type = "crew"
  definition = "crew.jsonc"
  ```

* Declarative flow projects must use a valid project-local definition path:

  ```toml
  [tool.crewai]
  type = "flow"
  definition = "flows/research.yaml"
  ```

* `Flow.from_definition(definition)` is removed. Use:

  ```python
  Flow.from_declaration(contents=definition)
  ```

* `FlowDefinition.to_json()` and `FlowDefinition.to_yaml()` are removed.
  Use `FlowDefinition.to_dict()` and serialize with the caller's JSON or
  YAML library.

* `FlowDefinition.from_dict()` is removed. Use:

  ```python
  FlowDefinition.from_declaration(contents=data)
  ```

* `FlowDefinition.json_schema()` is removed. Use Pydantic's schema API only
  where schema generation is intentionally needed:

  ```python
  FlowDefinition.model_json_schema(by_alias=True)
  ```

* `crewai_cli.run_crew.find_crew_json_file()` and `_has_json_crew()` are
  removed. Use `configured_project_json_crew()` or the shared
  `crewai_core.project.configured_project_definition("crew")` helper.

* `crewai reset-memories` now only loads JSON crews declared through
  `[tool.crewai].definition`, and invalid declared JSON crew definitions
  fail instead of silently falling back to classic crew discovery.

* Address code review comments

lib/cli/tests/test_create_crew.py

@@ -741,6 +741,10 @@ def test_json_create_provider_preselects_default_model(tmp_path, monkeypatch):
     assert pyproject["tool"]["hatch"]["build"]["targets"]["wheel"][
         "only-include"
     ] == ["agents", "crew.jsonc", "tools", "knowledge", "skills"]
+    assert pyproject["tool"]["crewai"] == {
+        "type": "crew",
+        "definition": "crew.jsonc",
+    }
 
     crew_template = (tmp_path / "json_crew" / "crew.jsonc").read_text()
     assert (