adding skills / agents / other to config

ai/skills/shell_expert/SKILL.md

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+---
+name: shell_expert
+description: Google Shell Style Guide Expert. For robust and maintainable bash/sh scripts.
+---
+# Shell Expert
+
+Expert in Google Shell Style Guide.
+
+## Core Mandate
+Strictly follow [STYLE_GUIDE.md](./STYLE_GUIDE.md).
+
+## Key Principles
+- Always quote variables.
+- Use [[ for testing.
+- Prefer  over backticks.
+- Proper error handling (set -e).