chore: track .claude/rules/ and add self-maintenance instruction

Un-ignore .claude/rules/ so path-scoped rule files are shared via git.
Add instruction in CLAUDE.md to update rule files when modifying classes,
controllers, endpoints, or metrics — keeps rules current as part of
normal workflow rather than requiring separate maintenance.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -38,7 +38,8 @@ Thumbs.db
 logs/
 
 # Claude
-.claude/
+.claude/*
+!.claude/rules/
 .superpowers/
 .playwright-mcp/
 .worktrees/