Refactor: centralize AI instructions and opencode config into common dotfiles

dotfiles/common/.config/ai/rules/safety.md

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+# Safety Rules
+
+Never break an existing host.
+
+- Identify host, layer, risk
+- Always test with --limit
+- Use dry-run (--check --diff)
+- Avoid destructive changes
+
+## Git Rules
+
+- Never auto-commit; always ask the user before committing