Add interval-based update mode for test dataset generation

Add optional interval-based random content updates to the cdssync
migration test dataset generator and document the new behavior.

This allows the dataset to be created once and then updated either
continuously or every N seconds while preserving the intended
special-case file structure.

cdssync/AGENTS.md

@@ -15,6 +15,10 @@ For migration test datasets in this workspace, follow this process by default:
 
 - Use `/home/aw/code/cds/cdssync/generate_migration_test_dataset.sh` to create the dataset unless the user explicitly asks for a different method.
 - Prefer `/home/aw/code/cds/cdssync/migration-test-dataset` as the local canonical dataset location unless the user specifies another target.
+- The generator script accepts an optional `UPDATE_INTERVAL_SECONDS` argument:
+  - omit it to create the dataset once and exit
+  - use `0` for continuous random content updates
+  - use any integer greater than `0` to rewrite mutable files every `N` seconds
 - If ACL/xattr coverage matters, ensure the generation host has:
   - `acl` installed for `setfacl` and `getfacl`
   - `attr` installed for `setfattr` and `getfattr`