chore(automation): tighten run.py + envelope.py docstrings

Re-apply the trim style after the prior refactor commit re-introduced
a multi-line docstring on AutomationRun.

- AutomationRun: drop the four-line docstring explaining where
  per-step session ids live; move the note to a single-line inline
  comment right above ``step_results`` where it's actionable.
- AutomationDefinition: drop the design-plan cross-reference; the
  module docstring already establishes what the file is.

No behaviour change.

surfsense_backend/app/automations/schemas/definition/envelope.py

@@ -12,7 +12,7 @@ from .trigger_spec import TriggerSpec
 
 
 class AutomationDefinition(BaseModel):
-    """Top-level shape of an automation. See automation-design-plan.md §5."""
+    """Top-level shape of an automation."""
 
     model_config = ConfigDict(extra="forbid")