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/lib.rs

@@ -67,6 +67,7 @@ pub mod summarizer;
 
 // Re-export main types for convenience
 pub use types::{
+    DecayResult,
     ExtractedMemory,
     ExtractionConfig,
     GrowthStats,
@@ -75,6 +76,7 @@ pub use types::{
     RetrievalConfig,
     RetrievalResult,
     UriBuilder,
+    effective_importance,
 };
 
 pub use extractor::{LlmDriverForExtraction, MemoryExtractor};