Write-time admission control for the knowledge store (#970). The cap +
importance-eviction is a backstop after the fact is written; admission is
the boundary before it, so a capped store fills with signal, not noise.
Applied only to direct ctx_knowledge remember (the agent-facing path);
internal restorers (archive rehydrate, cognition auto-promotion) bypass it.
Cluster compaction (#971): the background cognition loop collapses piles of
low-value, mutually-similar facts into a single recoverable digest, so a busy
store’s live fact count actually drops instead of churning at its cap. It is
strictly guarded — only faded (< max_confidence), barely-confirmed
(<= max_confirmations), never-frequently/recently-retrieved facts in a
cluster of at least min_cluster qualify — and lossless, since the originals
are archived and rehydrate on recall.