refactor: extract NextActions into reusable workflow-surface models
Move wrapper-draft next_actions from handler-local dict helpers into
typed models in workflow_surface/next_actions.py. Adds NextActionTool
enum, NextActionPatchExample, and NextActions.from_wrapper_hints()
constructor. Replaces private _wrapper_draft_next_actions and
_wrapper_draft_patch_examples helpers. Adds can_continue as additive
advisory field. Keeps backward-compatible aliases in models.py.