Add OpenStock agent CLI skill

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

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+# `openstock cache prune`
+
+Purpose: apply cache retention policy.
+
+Usage:
+
+```bash
+openstock cache prune --dry-run
+openstock cache prune
+```
+
+Reads: `~/.config/openstock/cache`.
+
+Deletes: old cache files unless `--dry-run` is set.
+
+Output fields: `dry_run`, `deleted_files`, `deleted_bytes`, `reports`.
+
+Agent rule: run `--dry-run` first unless the user explicitly requests cleanup.