feat(growth): add memory decay + time-weighted scoring + remove dead frontend

- Add effective_importance() with exponential time decay (30-day half-life)
  and access count boost for fair scoring of stale vs fresh memories
- Add SqliteStorage::decay_memories() for periodic maintenance:
  reduces stored importance per interval, archives (deletes) below threshold
- Update find() scoring to use time-decayed importance in sort
- Add DecayResult type and effective_importance re-export in lib.rs
- Remove dead frontend active-learning.ts (370 lines, zero imports)

crates/zclaw-growth/src/storage/sqlite.rs

@@ -270,6 +270,74 @@ impl SqliteStorage {
 
         Ok(())
     }
+
+    /// Decay stale memories: reduce importance for long-unaccessed entries
+    /// and archive those below the minimum threshold.
+    ///
+    /// - For every `decay_interval_days` since last access, importance drops by 1.
+    /// - Memories with importance ≤ `archive_threshold` are deleted.
+    pub async fn decay_memories(
+        &self,
+        decay_interval_days: u32,
+        archive_threshold: u8,
+    ) -> crate::types::DecayResult {
+        // Step 1: Reduce importance of stale memories
+        let decay_result = sqlx::query(
+            r#"
+            UPDATE memories
+            SET importance = MAX(1, importance - CAST(
+                (julianday('now') - julianday(last_accessed)) / ? AS INTEGER
+            ))
+            WHERE last_accessed < datetime('now', '-' || ? || ' days')
+              AND importance > 1
+            "#,
+        )
+        .bind(decay_interval_days)
+        .bind(decay_interval_days)
+        .execute(&self.pool)
+        .await;
+
+        let decayed = decay_result
+            .map(|r| r.rows_affected())
+            .unwrap_or(0);
+
+        // Step 2: Remove memories that fell below archive threshold
+        // and haven't been accessed in 90+ days
+        let archive_result = sqlx::query(
+            r#"
+            DELETE FROM memories
+            WHERE importance <= ?
+              AND last_accessed < datetime('now', '-90 days')
+            "#,
+        )
+        .bind(archive_threshold as i32)
+        .execute(&self.pool)
+        .await;
+
+        // Also clean up FTS entries for archived memories
+        let _ = sqlx::query(
+            r#"
+            DELETE FROM memories_fts
+            WHERE uri NOT IN (SELECT uri FROM memories)
+            "#,
+        )
+        .execute(&self.pool)
+        .await;
+
+        let archived = archive_result
+            .map(|r| r.rows_affected())
+            .unwrap_or(0);
+
+        if decayed > 0 || archived > 0 {
+            tracing::info!(
+                "[SqliteStorage] Memory decay: {} decayed, {} archived",
+                decayed,
+                archived
+            );
+        }
+
+        crate::types::DecayResult { decayed, archived }
+    }
 }
 
 impl sqlx::FromRow<'_, SqliteRow> for MemoryRow {
@@ -567,7 +635,7 @@ impl VikingStorage for SqliteStorage {
             scorer.is_embedding_available()
         };
 
-        let mut scored_entries: Vec<(f32, MemoryEntry)> = Vec::new();
+        let mut scored_entries: Vec<(f32, f32, MemoryEntry)> = Vec::new();
 
         for row in rows {
             let entry = self.row_to_entry(&row);
@@ -613,15 +681,18 @@ impl VikingStorage for SqliteStorage {
                 }
             }
 
-            scored_entries.push((semantic_score, entry));
+            // Apply time decay to importance before final scoring
+            let time_decayed_importance = crate::types::effective_importance(&entry);
+
+            scored_entries.push((semantic_score, time_decayed_importance, entry));
         }
 
-        // Sort by score (descending), then by importance and access count
+        // Sort by: semantic score → time-decayed importance → access count (all descending)
         scored_entries.sort_by(|a, b| {
             b.0.partial_cmp(&a.0)
                 .unwrap_or(std::cmp::Ordering::Equal)
-                .then_with(|| b.1.importance.cmp(&a.1.importance))
-                .then_with(|| b.1.access_count.cmp(&a.1.access_count))
+                .then_with(|| b.1.partial_cmp(&a.1).unwrap_or(std::cmp::Ordering::Equal))
+                .then_with(|| b.2.access_count.cmp(&a.2.access_count))
         });
 
         // Apply limit
@@ -629,7 +700,7 @@ impl VikingStorage for SqliteStorage {
             scored_entries.truncate(limit);
         }
 
-        Ok(scored_entries.into_iter().map(|(_, entry)| entry).collect())
+        Ok(scored_entries.into_iter().map(|(_, _, entry)| entry).collect())
     }
 
     async fn find_by_prefix(&self, prefix: &str) -> Result<Vec<MemoryEntry>> {