fix: honor explicit XDG directory overrides

[luochen211]

Problem

TeamDNA's path helpers only honor XDG_CONFIG_HOME and XDG_DATA_HOME on Linux. When either variable is explicitly set on macOS or Windows, the helpers ignore it and continue using ~/.teamdna.

This also breaks the MCP end-to-end test isolation on macOS: the tests provide temporary XDG directories, but the server reads the user's real home-directory configuration instead. On macOS this caused 5 of the 18 MCP tests to fail.

Change

• Honor an explicitly configured XDG directory before applying platform defaults.
• Preserve the existing defaults when no XDG override is present: ~/.config/teamdna and ~/.local/share/teamdna on Linux, and ~/.teamdna on macOS and Windows.
• Clarify the override behavior in the path helper documentation.

Validation

• npm ci
• npm test — 18 tests passed

Closes #15