feat: add program prompt template variable (#89)

This protects against typos, and helps keep usage consistent with the
template's name.

Author: mtth

src/git_draft/prompt.py

@@ -84,6 +84,7 @@ def _load_layouts() -> Mapping[str, str]:
 
 class _Context(TypedDict):
     prompt: Mapping[str, str]
+    program: PromptName
     toolbox: Toolbox
 
 
@@ -167,7 +168,7 @@ def render(self, args: Sequence[str]) -> str:
         try:
             opts = docopt.docopt(self._doc, list(args))
         except docopt.DocoptExit as exc:
-            raise ValueError(f"Invalid template arguments: {args}") from exc
+            raise ValueError(f"Invalid template arguments\n{exc}") from exc
         return self._template.render({**self._context, "opts": opts})
 
 
@@ -185,7 +186,11 @@ def _load_prompt(
     rel_path = Path(f"{name}.{_extension}")
     assert env.loader, "No loader in environment"
     template = env.loader.load(env, str(rel_path))
-    context: _Context = dict(prompt=_load_layouts(), toolbox=toolbox)
+    context: _Context = dict(
+        program=name,
+        prompt=_load_layouts(),
+        toolbox=toolbox
+    )
     try:
         module = template.make_module(vars=cast(dict, context))
     except jinja2.TemplateError as exc:

src/git_draft/prompts/add-docstrings.jinja

@@ -4,7 +4,7 @@
 Add docstrings to one or more public symbols
 
 Usage:
-  add-docstrings [--path PATH] [--symbol SYMBOL]
+  {{ program }} [--path PATH] [--symbol SYMBOL]
 
 Options:
   --path PATH      Path to (Python) file where docstrings should be added. If

src/git_draft/prompts/add-test.jinja

@@ -4,7 +4,7 @@
 Add tests for a symbol
 
 Usage:
-  add-test --symbol NAME [--src-path PATH] [--test-path PATH]
+  {{ program }} --symbol NAME [--src-path PATH] [--test-path PATH]
 
 Options:
   --symbol NAME     Name of the symbol to be tested.