Add OpenStock agent CLI skill

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

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+# `openstock cache`
+
+## Purpose
+
+Inspect and prune local cache under `~/.config/openstock/cache`.
+
+## Subcommands
+
+| Command | Purpose |
+| --- | --- |
+| `openstock cache status` | Show cache size and namespace summary. |
+| `openstock cache prune [--dry-run]` | Apply cache retention policy. |
+
+## IO
+
+| Direction | Data |
+| --- | --- |
+| File read | Cache directories. |
+| File delete | Old cache files unless `--dry-run` is used. |
+| External IO | none |
+| Side effect | `prune` deletes files unless dry-run. |
+
+## Output Fields
+
+| Field | Meaning |
+| --- | --- |
+| `root` | Cache root path. |
+| `exists` | Whether cache root exists. |
+| `total_files` | Number of cache files. |
+| `total_bytes` | Total cache size. |
+| `namespaces` | Cache namespace summaries. |
+| `dry_run` | Whether prune only reported actions. |
+| `deleted_files` | Number of deleted files. |
+| `deleted_bytes` | Bytes removed. |
+| `reports` | Prune reports by namespace. |
+
+## Agent Notes
+
+Run `cache prune --dry-run` before destructive pruning unless the user clearly asks to clean cache.

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.

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.