style(smolmachines): provision_git stub uses pass not del

Addresses PR #69 review comment: `del plan, target` was just a
silence-the-unused-arg gesture but reads oddly for a stub. `pass`
is the standard "this is a stub" sentinel.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/smolmachines/backend.py

@@ -53,7 +53,7 @@ class SmolmachinesBottleBackend(
         # Chunk 4 follow-on: needs the git-gate inner Plan (so the
         # gitconfig insteadOf URL points at the gate's host) and
         # the agent image must contain `git`. Stub for chunk 4a.
-        del plan, target
+        pass
 
     def prepare_cleanup(self) -> SmolmachinesBottleCleanupPlan:
         return SmolmachinesBottleCleanupPlan()