Publish only the work: leave build artifacts out and report them (#80)

The clone gets the standard build artifacts in its own exclude file, so
they never show as changes, even to a model's `git add -A`. The
harness's commits leave the same patterns out even when the project
tracks them, as the sandbox's main did. What was left out is reported in
the publish note and the PR description. Found again in bring-up run 3
(sandbox PR #208).

Fixes #80.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

nisse-dev/src/nisse_dev/agent.py

@@ -217,6 +217,7 @@ def prepare_workspace(
     if not repo.exists():
         repo.clone(remote, credential=credential)
     repo.configure_identity(account, f"{account}@users.noreply.local")
+    repo.install_excludes()  # build artifacts are never the work (#80)
     return repo