Adds `bd dep tree --format mermaid` to export dependency trees as Mermaid.js flowcharts. Features: - Status indicators: ☐ open, ◧ in_progress, ⚠ blocked, ☑ closed - Theme-agnostic design - Works with --reverse flag - Comprehensive unit tests following TDD Co-authored-by: David Laing <david@davidlaing.com>