Bug: Base Builder MCP still fetches documentation from archived base/web path

## Summary

`base-builder-mcp` still resolves Base documentation against the old `base/web` repository structure.

The current Base documentation is maintained in `base/docs`.

As a result, valid Base documentation URLs can be converted into obsolete raw GitHub paths and return `404 Not Found`.

## Affected files

- `sidebar.ts`
- `tools.ts`

## Problem

`sidebar.ts` fetches the documentation sidebar from the legacy repository:

```text
https://raw.githubusercontent.com/base/web/refs/heads/master/apps/base-docs/sidebar.ts
```

`tools.ts` also constructs guide URLs using `base/web`:

```ts
const guidePath = guideLink.replace("https://docs.base.org", "");

const githubRawUrl =
`https://raw.githubusercontent.com/base/web/refs/heads/master/apps/base-docs/docs/pages${guidePath}.mdx`;
```

However, `base/web` is archived and the current documentation repository is:

```text
https://github.com/base/docs
```

## Steps to reproduce

Use this current Base documentation URL:

```text
https://docs.base.org/get-started/docs-mcp
```

`getGuide()` converts it into:

```text
https://raw.githubusercontent.com/base/web/refs/heads/master/apps/base-docs/docs/pages/get-started/docs-mcp.mdx
```

That legacy path does not contain the current guide.

The guide currently exists in the active documentation repository at:

```text
base/docs/docs/get-started/docs-mcp.mdx
```

## Expected behavior

A valid page published on `docs.base.org` should be successfully retrieved by the `BuildOnBase` MCP tool.

## Actual behavior

The MCP maps current Base documentation URLs to paths under the archived `base/web` repository.

This can result in `404 Not Found` for valid current documentation.

## Impact

AI agents using Base Builder MCP may:

- fail to retrieve valid Base documentation;
- receive 404 errors for existing guides;
- use stale documentation;
- miss newly added Base features;
- fall back to an outdated embedded sidebar;
- incorrectly conclude that a valid guide does not exist.

This makes the MCP unreliable as an authoritative Base documentation source.

## Suggested fix

Update documentation resolution to use the current Base docs repository:

```text
https://github.com/base/docs
```

The guide resolver should no longer construct URLs under:

```text
base/web/apps/base-docs/docs/pages/
```

The hardcoded fallback sidebar should also be updated or replaced with a source that follows the current documentation structure.

## Suggested tests

Add regression tests for current Base documentation URLs.

For example:

```text
https://docs.base.org/get-started/docs-mcp
```

The test should verify that:

- the resolved source exists;
- the request does not return 404;
- the returned document is not empty;
- current `docs.base.org` routes map to the active documentation repository.

Start with sidebar.ts and tools.ts, especially getGuide() and the current sidebar and guide URL construction. Trace the https://docs.base.org/get-started/docs-mcp route against the active base/docs repository, then add regression coverage for a successful, non-empty response without a 404. Done means current documentation URLs resolve through the active repository and the fallback sidebar no longer depends on the archived base/web path.

Written by the indexing model from the issue text.