Core scaffolding — EngineRestBitBucket, GitHostBitBucket, set_bitbucket_host()

[maciekbanas]

Part of #754.

Add the foundational classes and user-facing function for BitBucket Cloud support:

• EngineRestBitBucket — REST engine targeting api.bitbucket.org/2.0, with BitBucket-style cursor pagination (next URL), workspace/repo listing, and commit retrieval.
• GitHostBitBucket — Host subclass (REST-only, no GraphQL). Overrides org checking to use workspace endpoints, repos/commits retrieval via REST, and raises informative errors for not-yet-supported methods.
• set_bitbucket_host() — Exported function + GitStats$set_bitbucket_host() method, following the same pattern as set_github_host() / set_gitlab_host().
• Test helpers: GitHostBitBucketTest, TestEngineRestBitBucket, create_bitbucket_testhost().
• Update DESCRIPTION (mention BitBucket), NAMESPACE, error messages, and NEWS.md.