[IMP] add migration log file

to store all the migration logs. File reset at each migration.

README.md

@@ -413,10 +413,23 @@ The script works on a **copy** of the original database. You can restart as many
 
 ### Viewing Detailed Logs
 
-Odoo/OpenUpgrade logs are displayed in real-time. For a problematic migration:
+All output (scripts, Docker containers, OpenUpgrade logs) is automatically saved to `migration.log` at the project root. The file is overwritten at the start of each run (full migration or resume).
+
+```bash
+# Follow live output while also saving to file (already done automatically)
+tail -f migration.log
+
+# Search for errors after the fact
+grep -i error migration.log
+
+# View with ANSI colors preserved
+less -R migration.log
+```
+
+For a problematic migration:
 
 1. Note the version where the error occurs
-2. Check the logs to identify the problematic module/table
+2. Check `migration.log` to identify the problematic module/table
 3. Add a fix in the `pre_upgrade.sh` for that version
 4. Restart the migration