query by NodeType instead of key prefix

Replace key prefix matching (journal#j-, daily-, weekly-, monthly-)
with NodeType filters (EpisodicSession, EpisodicDaily, EpisodicWeekly,
EpisodicMonthly) for all queries: journal-tail, digest gathering,
digest auto-detection, experience mining dedup, and find_journal_node.

Add EpisodicMonthly to NodeType enum and capnp schema.

Key naming conventions (journal#j-TIMESTAMP-slug, daily-DATE, etc.)
are retained for key generation — the fix is about how we find nodes,
not how we name them.

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

schema/memory.capnp

@@ -45,6 +45,7 @@ enum NodeType {
   episodicDaily   @1;
   episodicWeekly  @2;
   semantic        @3;
+  episodicMonthly @4;
 }
 
 enum Provenance {