Add OpenStock agent CLI skill

skills/openstock-agent-cli/references/commands/cache/status/guide.md

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+# `openstock cache status`
+
+Purpose: inspect local cache size and namespace summary.
+
+Usage:
+
+```bash
+openstock cache status
+```
+
+Reads: `~/.config/openstock/cache`.
+
+Output fields: `root`, `exists`, `total_files`, `total_bytes`, `namespaces`.
+
+Agent rule: use before cache cleanup or when diagnosing stale local data.