fix(mcp): default get_module_tree output_dir to repo docs path

[BrahmendraBachi]

Summary

Fixes the default output_dir resolution in the legacy get_module_tree MCP tool so it looks for module_tree.json under the target repository's docs folder instead of the MCP server's current working directory.

What changed

In _legacy_get_module_tree, when output_dir is not provided:

• Before: defaulted to Path("docs").resolve(), which resolved relative to the MCP process cwd
• After: defaults to (repo_path / "docs").resolve(), so docs are always resolved relative to the analyzed repo

When output_dir is explicitly passed, behavior is unchanged — the provided path is still expanded and resolved as before.

Why

When CodeWiki runs as an MCP server in Cursor (or another IDE), the process cwd is often the workspace root or the MCP install directory, not the repo being analyzed. That caused get_module_tree to look in the wrong place (e.g. ./docs/module_tree.json in the IDE workspace) even when docs had been generated under <repo>/docs/.

This aligns the legacy tool with the intended convention that generated docs live next to the repo unless an explicit output_dir is given.

Test plan

• Call get_module_tree with only repo_path (no output_dir) for a repo that has <repo>/docs/module_tree.json — should succeed
• Call get_module_tree from a different cwd (e.g. MCP server started outside the repo) — should still find the module tree
• Call get_module_tree with an explicit output_dir — should still use the provided path
• Call get_module_tree when no module tree exists — should return the same clear error as before