Allow specify workflow run to execute YAML files without a project (#2825)

* Initial plan

* feat: add --workflow option to init command for post-init workflow execution

* chore: remove unused import in test file

* refactor: allow workflow run without project when given a YAML file path

Instead of adding --workflow to init, make `specify workflow run ./file.yml`
work without requiring a .specify/ project directory. When the source is a
YAML file that exists on disk, cwd is used as the project root. When it's a
workflow ID, the .specify/ project requirement is preserved.

* Handle standalone workflow path edge cases

* Fix USERPROFILE env var portability and docs notation

* Fix workflow YAML path detection to require regular files

* Harden workflow run against unsafe .specify paths

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

src/specify_cli/workflows/engine.py

@@ -449,10 +449,10 @@ class WorkflowEngine:
         ValueError:
             If the workflow YAML is invalid.
         """
-        path = Path(source)
+        path = Path(source).expanduser()
 
         # Try as a direct file path first
-        if path.suffix in (".yml", ".yaml") and path.exists():
+        if path.suffix.lower() in (".yml", ".yaml") and path.is_file():
             return WorkflowDefinition.from_yaml(path)
 
         # Try as an installed workflow ID