Add automatic detection and merging of stacked pull requests with comprehensive conflict checking and user confirmation. When merging a PR that's part of a stack, the CLI now: - Auto-detects stack membership via stack_id field - Displays preview of all PRs to be merged (current + below) - Checks cumulative patches for conflicts before merging - Prompts user for confirmation before executing merge - Provides clear error messages for conflicts API changes: - Add stack fields to Pull: stack_id, change_id, parent_change_id - Add MergeCheckRequest/Response types for conflict detection - Add merge_check() method to TangledClient CLI changes: - Refactor merge() to detect and handle stacked PRs - Add helper functions for stack ordering and conflict checking - Maintain backward compatibility for non-stacked PRs