Implements branch lifecycle management for conversational branching:
create_root_branch, create_fork, switch_branch, mark_branch_status,
revive_branch, get_branch_tree, and build_cross_branch_context.
Five integration tests cover the full lifecycle from root creation
through forking, switching, dead-end marking, and tree retrieval.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
