Track .claude/rules agent instructions (fix gitignore)

Allow version-controlled agent rule files under .claude/rules/ while keeping
machine-local Claude Code state ignored. Aligns with state-hub gitignore pattern.

.claude/rules/stack-and-commands.md

@@ -0,0 +1,19 @@
+## Stack
+
+<!-- TODO: Fill in language, frameworks, and key dependencies -->
+- **Language:**
+- **Key deps:**
+
+## Dev Commands
+
+```bash
+# TODO: Fill in the standard commands for this repo
+
+# Install dependencies
+
+# Run tests
+
+# Lint / type check
+
+# Build / package (if applicable)
+```