feat(custom-property): add values_editable_by support

[gdams]

Summary

Adds the values_editable_by field to organization custom properties. This controls which actors can update a property's value on individual repositories once it has been defined at the org level.

Allowed values

• org_actors — only organization actors can update the value
• org_and_repo_actors — both organization and repository actors can update
• null (default) — not settable at the repository level

Example

settings+: {
  custom_properties+: [
    orgs.newCustomProperty('cost_center') {
      value_type: 'string',
      required: true,
      default_value: 'platform',
      values_editable_by: 'org_and_repo_actors',
    },
  ],
},

Changes

• otterdog/models/custom_property.py — new values_editable_by: str | None field with enum validation.
• examples/template/otterdog-defaults.libsonnet — default values_editable_by: null on newCustomProperty.
• otterdog/resources/schemas/custom-property.json — anyOf enum-or-null schema.
• tests/models/test_custom_property.py + tests/providers/github/integration/test_org_custom_properties.py — updated to cover the new field.

Testing

• poetry run pytest tests/ → 243 passed, 2 skipped, 0 failures
• poetry run mypy otterdog → clean
• poetry run ruff check / ruff format --check → clean