feat: deterministic compaction summary from the persisted goal record

buildCompactionContext now folds in a reproducible progress summary
(recent checkpoints + recent lifecycle events) reconstructed from the
goal's persisted checkpoints/history via a new
buildCompactionProgressSummary helper, instead of relying on chat memory,
and labels it as such. Adds unit tests for windowing, determinism, the
empty case, and inclusion in the compaction context. Item 6.3.