[codex] Fix product guide storage paths

[cloudyun888]

Summary

• Update the assistant product guide to describe per-instance storage under <workspace>/.openacp/.
• Keep shared global paths only for shared downloads/caches such as ~/.openacp/agents/ and ~/.openacp/cache/registry-cache.json.
• Correct the config, agent store, log, and data-storage path references in the guide.

Why

The product guide is injected into the Assistant prompt, so stale paths can make the Assistant give users outdated configuration and storage advice. The current instance model stores config, sessions, logs, plugins, and instance agent definitions in the instance root, while only shared downloads/caches live under ~/.openacp/.

Validation

• node --check src/data/product-guide.ts
• git diff --check
• rg -n "Config file:|Installed agent definitions|logging\\.logDir|Session-specific logs|Data & Storage|OpenACP stores per-instance|~/.openacp/config\\.json|~/.openacp/logs|~/.openacp/agents\\.json|All data is stored in" src/data/product-guide.ts
• Cross-checked against src/core/instance/instance-context.ts, src/core/instance/__tests__/instance-context.test.ts, and the existing GitBook storage/config docs.

Note: I did not run the full test suite because this checkout does not have node_modules installed; this change is limited to the static product-guide string and was syntax-checked with Node.