Cap remote compact overflow retries

[cbusillo]

Summary

• replace the 32-step remote compaction context-overflow trim loop with one bounded retry
• trim to a recent-history window before retrying, then fall back to emergency compaction if it still overflows
• add focused tests for the bounded trim behavior

Why

The old path could issue one failed remote compaction request per dropped history item, up to 32 attempts. That is the likely source of the repeated remote compaction failures and avoidable rate-limit burn we saw during dogfood.

Validation

• cargo test -p code-core compact_remote -- --nocapture
• ./build-fast.sh