fix: derive MCP navigation from current Base docs config

[huklaa]

Addresses #16

This PR keeps the MCP navigation/sidebar in sync with the active Base docs source.

What changes

• Navigation now comes from base/docs/docs/docs.json (replacing the archived base/web sidebar fetch).
• Derives a compact navigation view from the current Base docs navigation tree, supporting tabs, groups, pages, global, and anchors.
• Filters external global.anchors so only docs.base.org URLs are exposed as guideLink options. Mintlify global anchors also include external destinations (e.g. Status, Blog, Explorer, GitHub); advertising those as guide choices could make the model pick a guideLink that getGuide/getStepsList cannot resolve, since that tool only accepts Base documentation URLs.
• Removes the stale hardcoded sidebar fallback.
• Replaces it with a minimal fallback that points to docs.base.org when the remote config cannot be fetched or parsed.
• Routes refresh diagnostics to stderr.
• Adds an npm test script that compiles TypeScript and runs the sidebar tests with Node's built-in test runner.

Tests

Existing tests cover navigation and global anchors (including nested tabs/groups/pages, malformed nodes, and missing navigation), and now assert that unsupported external anchors are excluded while docs.base.org anchors are kept as guide choices.

Why this is distinct from #12

#12 handles guide file retrieval and candidate path resolution from base/docs.
This PR intentionally leaves that resolver alone and covers the separate navigation-sync part of #16.

Verification

• npx tsc --noEmit
• npm test
• git diff --check