feat(core): validate deferred_apply_threshold < compaction_threshold in wizard

[bug-ops]

Follow-up from epic #1294 review (IMPL-CRIT-01).

The --init wizard validates that deferred_apply_threshold is in range (0, 1) but does not enforce that it is strictly less than compaction_threshold. A user setting deferred_apply_threshold = 0.85 with compaction_threshold = 0.80 would cause Tier 0 to never fire before Tier 1, defeating the purpose of deferred summarization.

Note: the CRIT-01 safety net in compact_context() prevents data loss even in this misconfiguration. But the tier ordering would be effectively broken.

Fix

In src/init.rs build_config() / wizard step:

1. After collecting both values, validate deferred < compaction
2. If not, emit a clear error/warning: "deferred_apply_threshold must be less than compaction_threshold"
3. Ideally, prompt the user to re-enter the value rather than just warning

Also add a runtime check at agent startup with tracing::warn! if the invariant is violated.