Add workspace change watching and PR test workflow

[zac]

Summary

Adds Workspace.watchChanges(at:recursive:) and ChangeEvent so callers can observe future workspace mutations on files and directory trees. This also adds a PR test workflow and reorganizes the new tests around functionality rather than line-coverage buckets.

Changes

• add the ChangeEvent model and a watcher registry to Workspace
• emit buffered change events for writes, directory creation, deletes, copies, moves, applyEdits, and applyReplacement
• deliver recursive directory, file, and symlink events, including move matching by both destination and source paths
• add a PR GitHub Actions workflow that runs swift test, publishes xUnit and coverage summaries, and uploads artifacts
• expand and retag the functional test suites across in-memory, overlay, mountable, sandbox, and security-scoped filesystems

Testing Performed

• swift test
• swift test --enable-code-coverage

Coverage Summary

• overall: 93.97% region coverage, 97.18% line coverage
• Workspace.swift: 94.87% region coverage
• SecurityScopedFilesystem.swift: 91.46% region coverage
• ChangeEvent.swift: 100% region coverage
• no covered source files are below 80% region coverage

Screenshots

• N/A (no UI changes)