Add OpenStock agent CLI skill

skills/openstock-agent-cli/references/commands/dart/document/guide.md

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+# `openstock dart document`
+
+Purpose: fetch and extract text from a DART disclosure document by receipt number.
+
+Usage:
+
+```bash
+openstock dart document 20260609000000 --max-chars 20000
+```
+
+Inputs: `rcept_no`, optional `--force`, `--max-chars`.
+
+Reads: `OPENDART_API_KEY`.
+
+Writes: document ZIP cache.
+
+Output fields: document metadata and extracted text.
+
+Agent rule: use `--max-chars` to cap large documents.