docs: add doc.go and expand README for V1

Document lifecycle concepts, companion packages, and API reference.
Add gitignore for coverage artifacts.

Co-authored-by: Cursor <cursoragent@cursor.com>

.gitignore

@@ -0,0 +1,6 @@
+# Test coverage
+coverage.out
+coverage.html
+
+# Go build artifacts
+*.test