Add OpenStock agent CLI skill

skills/openstock-agent-cli/references/commands/update/guide.md

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+# `openstock update`
+
+## Purpose
+
+Update the installed `openstock` binary from the latest Gitea release asset.
+
+## Usage
+
+```bash
+openstock update
+openstock update --force
+```
+
+## Input
+
+| Input | Meaning |
+| --- | --- |
+| `--force` | Reinstall from the latest release asset even when the current version matches. |
+| `OPENSTOCK_INSTALL_DIR` | Optional install directory override. |
+| `OPENSTOCK_RELEASE_API_URL` | Optional release API URL override. |
+| `OPENSTOCK_RELEASE_ASSET_SUFFIX` | Optional asset suffix override. |
+
+## IO
+
+| Direction | Data |
+| --- | --- |
+| External IO | Gitea latest release API and selected release asset download. |
+| File write | Installed `openstock` binary. |
+| Side effect | Replaces the installed binary when newer or forced. |
+
+## Output Fields
+
+| Field | Meaning |
+| --- | --- |
+| `release_api_url` | Gitea release API endpoint used for update discovery. |
+| `current_version` | Version of the running binary. |
+| `latest_version` | Version parsed from latest release tag. |
+| `release_tag` | Latest release tag. |
+| `release_url` | Human-readable release page. |
+| `asset_name` | Release asset selected for installation. |
+| `asset_url` | Asset download URL. |
+| `install_dir` | Directory where the binary is installed. |
+| `status` | `updated` or `up_to_date`. |
+| `stdout` | Installer stdout. |
+| `stderr` | Installer stderr. |
+
+## Agent Notes
+
+Use `--force` only when the user asks to reinstall or when verifying release assets.