Bug: `product-self-knowledge` skill falls through to open web search when docs URL fetch fails

## Bug: `product-self-knowledge` skill falls through to open web search when docs URL fetch fails

**Describe the issue**
When the skill's primary docs map URL is not directly fetchable (e.g. due to referrer/permissions constraints on the web fetch tool), there is no fallback that constrains search to official Anthropic documentation. The result is that Claude falls through to open web search and may source answers about Anthropic's own products from community blogs or third-party sites rather than official docs.

**Steps to reproduce**
1. Ask Claude a question about a Claude Code feature (e.g. `/ultrareview` / `/code-review ultra`)
2. Skill routes correctly to `https://docs.anthropic.com/en/docs/claude-code/claude_code_docs_map.md`
3. Direct fetch fails with a permissions/referrer error
4. Claude falls back to open `web_search` with no domain restriction
5. Answer is sourced from third-party blogs instead of `docs.claude.com` or `code.claude.com`

**Expected behavior**
If the primary docs URL fetch fails, the skill should instruct Claude to search within official Anthropic documentation domains (e.g. `site:docs.claude.com` or `site:code.claude.com`) before falling back to open web search.

**Proposed fix**
Add a fallback instruction to the skill along the lines of:
> If direct fetch of the docs map fails, search `docs.claude.com` or `code.claude.com` explicitly before falling back to open web search.

**Why this matters**
Customers asking about Anthropic's own products should receive answers sourced from official documentation. A correct answer from a third-party blog is still a weaker and less trustworthy foundation than the same answer from official docs — and in cases where the blog is wrong, there is no signal to Claude to be skeptical.