refactor(rules): group focused security rules

shared/rules/security/index.md

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+# Security Baseline
+
+Apply this short baseline to every code, configuration, infrastructure, and review task. Load only the focused security rules that match the affected boundary; their triggers are listed in the global instructions.
+
+* Treat external input as untrusted and validate it at the relevant trust boundary.
+* Use secure defaults, least privilege, explicit authorization, and fail-closed behavior.
+* Never hardcode, commit, log, expose, or weaken protections for secrets and credentials.
+* Use established platform security mechanisms. Do not invent cryptography or bypass validation, authentication, authorization, or TLS checks.
+* Keep dependencies minimal and trusted; avoid disclosing sensitive data or internal details in errors and diagnostics.