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>

src/query.rs

@@ -196,6 +196,7 @@ fn node_type_label(nt: NodeType) -> &'static str {
         NodeType::EpisodicSession => "episodic_session",
         NodeType::EpisodicDaily   => "episodic_daily",
         NodeType::EpisodicWeekly  => "episodic_weekly",
+        NodeType::EpisodicMonthly => "episodic_monthly",
         NodeType::Semantic        => "semantic",
     }
 }