fix: use future dates in checkpoint prune tests to prevent time-dependent failures

[alex-clawd]

Problem

TestPruneSqlite::test_older_than and TestPruneJson::test_older_than use hardcoded 20260417T000000 timestamps for the 'new' checkpoint. Once April 17 passes, that checkpoint is older than 1 day and gets pruned along with the 'old' one, causing assert count >= 1 to fail (count=0).

This is currently breaking CI on every PR branch.

Fix

Use 20990101T000000 for the 'new' checkpoint so the tests remain stable regardless of when they run.

Testing

The fix is a 2-line change in test fixtures. No production code changed.

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Note

Low Risk
Low risk: updates only test fixtures to avoid date-dependent failures; no production logic changes.

Overview
Fixes flaky checkpoint prune tests by changing the “new” checkpoint timestamps in test_checkpoint_cli.py to a far-future date so older_than=1 day pruning always leaves at least one checkpoint remaining regardless of when CI runs.

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