Add CLAUDE.md with fork sync instructions (#14)

## Summary
- Add CLAUDE.md with instructions for syncing fork with upstream
- Documents correct PR creation targeting verkada repo (not upstream astral-sh)

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Author: nickvines

CLAUDE.md

@@ -0,0 +1,40 @@
+# python-build-standalone (Verkada Fork)
+
+This is Verkada's fork of `astral-sh/python-build-standalone`.
+
+## Syncing with Upstream
+
+This repo has two remotes:
+- `origin`: `verkada/python-build-standalone` (our fork)
+- `upstream`: `astral-sh/python-build-standalone` (source repo)
+
+To sync with upstream:
+
+```bash
+# Fetch latest from upstream
+git fetch upstream
+
+# Create a branch for the sync (main is protected)
+git checkout main
+git checkout -b nickvines/sync-upstream-main
+
+# Merge upstream changes
+git merge upstream/main --no-edit
+
+# Push and create PR
+git push -u origin nickvines/sync-upstream-main
+```
+
+## Creating PRs
+
+**IMPORTANT**: Always create PRs against `verkada/python-build-standalone`, NOT the upstream repo.
+
+```bash
+# Correct - targets our fork
+gh pr create --repo verkada/python-build-standalone --title "Title" --body "Body"
+
+# WRONG - would target upstream
+gh pr create --title "Title" --body "Body"
+```
+
+The default `gh pr create` may target the upstream repo since this is a fork. Always explicitly specify `--repo verkada/python-build-standalone`.