docs(changelog): add project delete deleted_at field entry for PR #6639

Document the ISO-8601 deleted_at timestamp added to agents project delete JSON/YAML output in the changelog entry added for Cycle 12.

ISSUES CLOSED: #6314

CHANGELOG.md

@@ -147,6 +147,11 @@ The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
   `sandbox_root=.cleveragents/sandbox/`, so LLM file output (`FILE:` blocks)
   is written to disk during the execute phase. (#4222)
 
+- **`agents project delete` JSON/YAML `deleted_at` field** (#6639): The JSON and
+  YAML output of `agents project delete` now includes a `deleted_at` ISO-8601
+  timestamp in the `deletion_summary` object, making it easier to audit when a
+  project was removed.
+
 - **Robot Framework TDD Listener Guards** (#5436): Added three guard conditions to the
   `tdd_expected_fail_listener` `end_test()` function to prevent blindly inverting ALL test
   failures to passes, which was masking infrastructure errors and causing flaky CI behavior.