feat(phase9): write_text_file_cmd

Thin UTF-8 writer used by the new save-dialog path. Caller owns path
safety; the source path is always OS-dialog-provided. Two unit tests:
roundtrips a small UTF-8 string with non-ASCII chars and asserts a
nonexistent parent path returns an actionable error.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/src/commands/mod.rs

@@ -2,6 +2,7 @@ pub mod audio;
 pub mod clipboard;
 pub mod diagnostics;
 pub mod feedback;
+pub mod fs;
 pub mod hardware;
 pub mod hotkey;
 pub mod intentions;