#6 Add Games.export_bookmarks

[nstillman-te]

Part of #6.

Implements GET /api/games/export/bookmarks: client.games.export_bookmarks(**params) streams the authenticated user's bookmarked games (PGN or NDJSON). Requires OAuth2. Same pattern as export_by_player (params: since, until, max, moves, tags, clocks, evals, etc.). Tests use requests-mock (endpoint requires auth). README.rst and CHANGELOG.rst updated.

Checklist when adding a new endpoint

• Added new endpoint to the README.rst
• Ensured that my endpoint name does not repeat the name of the client. Wrong: client.users.get_user(), Correct: client.users.get() — method is export_bookmarks
• Typed the returned JSON using TypedDicts in berserk/types/ — N/A: response is a stream of existing Game objects (same as export_by_player), no new types
• Written tests for GET endpoints not requiring authentification — endpoint requires auth; added requests-mock test for path and query params (see CONTRIBUTING.rst)
• Added the endpoint to CHANGELOG.rst in the To be released section