0.4.2

This patch release adds external subcommands and improves automation and
release workflows without changing the board data format or breaking the
existing CLI contract.

Added

• Added external subcommand binaries with nested command discovery, argument
  forwarding, exit-code propagation, and versioned contract environment
  variables.
• Added escaped literal placeholders to sequential pinto automate plans.
• Added versioned Book deployments and a tag-driven GitHub Release workflow
  that derives release notes from CHANGELOG.md.

Fixed

• Fixed CI and dependency-policy validation for syn 3 dependency updates.
• Improved Windows compatibility for external command dispatch.