patchwork 0.2.0

• Rebuilt Patchwork around a library-first programmable model with CLI commands
  as thin adapters.
• Replaced patch sessions, baselines, stores, manifests, and target syntax with
  the v0.2 paths: .patchwork/<pkg>@<version>/,
  patches/<pkg>@<version>.patch, and
  .dart_tool/patchwork/<pkg>@<version>/.
• Added patchwork commit [pkg] and removed patchwork patch --commit.
• Added patchwork undo <pkg> for safe removal of lock-owned
  pubspec_overrides.yaml entries and generated applied directories.
• Added patchwork patch <pkg> [--continue [version]] for explicit patch
  carry-forward across dependency upgrades.
• Reworked patchwork.lock into a v2 lockfile with source sha256,
  patch.commit-sha256, and applied.patch-sha256 records.
• Added patch history commit-sha256 records so --continue <version> can
  safely reuse older patch files after a dependency upgrade.
• Removed obsolete historical patch files when an unchanged fresh edit proves
  the upgraded dependency source already contains the fix.
• Added source records for hosted, custom hosted, path, and git dependencies.