agent: code-atomiser-fix — migrate Tauri app_data_dir on bundle-identifier change

Commit 14313cf changed the Tauri bundle identifier from
uk.co.corbel.magnotia to consulting.corbel.lumotia. Tauri 2 keys both
app_data_dir and the webview's data store (localStorage, IndexedDB,
cookies, service worker, cache) plus all Tauri-plugin state files
(window-state geometry, autostart enable flag) by the identifier.
Without an explicit migration, every user's webview state was
silently orphaned on first launch under the new identifier; the
JS-side migrateLocalStorageKey helper introduced in 1608109 ran
against an empty store and no-op'd.

Fix: in the Tauri setup hook, before webview navigation, resolve the
legacy app_data_dir under the OLD bundle identifier and recursively
copy it to the new identifier's app_data_dir via an atomic staging
rename. Idempotent: legacy preserved as a backup; if both paths
exist post-rename, a warning is logged and the new path is used
unchanged.

Regression tests cover the migrate-only, both-exist, no-legacy, and
idempotent (run twice) cases against synthetic legacy/new paths.

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

crates/core/src/paths.rs

@@ -471,7 +471,12 @@ fn is_cross_device(err: &std::io::Error) -> bool {
     false
 }
 
-fn copy_dir_recursive(from: &Path, to: &Path) -> Result<(), std::io::Error> {
+/// Symlink-aware recursive directory copy used by the legacy data-dir
+/// migration and by the Tauri-side `app_data_dir` bundle-identifier
+/// migration in `src-tauri/src/tauri_app_data_migration.rs`. Exposed as
+/// `pub` so callers in sibling crates can reuse the same hardened
+/// implementation (see commit history for the symlink-loop defence).
+pub fn copy_dir_recursive(from: &Path, to: &Path) -> Result<(), std::io::Error> {
     std::fs::create_dir_all(to)?;
     for entry in std::fs::read_dir(from)? {
         let entry = entry?;