Add MCP Bundle (.mcpb) install documentation

[jeffreyaven]

Documents the prebuilt MCP Bundle (.mcpb) install path for Claude Desktop, a requirement for the Anthropic Desktop Extensions directory submission.

• docs/command-line-usage/mcp - new "Claude Desktop (one-click bundle)" subsection under "Integration with AI assistants", positioned as the recommended path: per-platform download table (releases/latest/download URLs, so no touch-ups needed on release bumps), install steps, checksum verification, and the Official MCP Registry listing (io.github.stackql/stackql-mcp). The existing claude_desktop_config.json instructions are reframed as "Manual configuration" for users who already have stackql installed.
• docs/installing-stackql - new "Claude Desktop / MCP" tab alongside the existing install methods, linking to the MCP page section (single source of truth for the download table).

Verified with a full production build: no broken links, anchor renders, .md companions and llms.txt emitted as expected.

🤖 Generated with Claude Code

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