Replace dart:io File/Directory with package:file abstractions so that
tests can use MemoryFileSystem instead of mutating the process-global
Directory.current.
- Add file: ^7.0.1 to core and kanban dependencies
- ProjectContext.find() accepts FileSystem fs parameter
- TicketStore, KanbanInitHook, InitCommand, all kanban commands accept
FileSystem fs (defaulting to LocalFileSystem())
- KanbanCommand and registerCommands() thread fs to subcommands
- Tests rewritten to use MemoryFileSystem() — no Directory.current mutation
- Remove dart_test.yaml (concurrency: 1 no longer needed)
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>