Add file-change monitoring to the kanban TUI so that tasks added via CLI
(or any external process) while the TUI is running are automatically
merged into the live view instead of being silently overwritten on save.
The implementation has three layers:
1. New FileAge built-in (compiler + runtime): returns the file's mtime as
Int64 via stat(), or -1 if the file does not exist. Follows the same
pattern as FileExists — symbol table registration, semantic analysis,
codegen, and POSIX ABI stub.
2. Merge-aware TBoard (kanban.data): tracks last-known mtime, deleted IDs
(so re-reads don't resurrect user-deleted tasks), and provides
HasExternalChanges/MergeFromDisk. Save() now auto-merges before
writing. ID conflicts (same ID, different task) are resolved by
reassigning the in-memory task to a fresh ID.
3. Idle polling in TKanbanUI.Run: every ~2 seconds (20 idle ReadKey
cycles at VTIME=1), checks HasExternalChanges and merges new tasks
with a status bar notification.
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