docs: add atomic-commit command documentation

Author: d-oit

.opencode/command/atomic-commit.md

@@ -0,0 +1,5 @@
+---
+description: Create atomic commits for staged changes
+agent: atomic-commit-creator
+---
+Analyze the current staged changes, identify logical units of work, and propose splitting them into atomic commits with descriptive messages. Use git diff --cached to examine changes and suggest selective staging if needed.