feat(memory): temporal decay for memory entries

**Context:** ROADMAP.md → Memory security → Temporal decay

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## Component

Agent (Python runtime)

## Describe the feature

Configurable **per-entry TTL** with faster decay for unverified content. Expired entries excluded from retrieval (soft-delete or archive).

## Use case

Repo conventions change; year-old review-derived rules may be wrong. Unverified single-source memories should not persist at full weight indefinitely.

## Proposed solution

1. TTL fields on write; decay curve configurable per source type.
2. Background sweeper or lazy expiry at read time.
3. Operator override to pin high-trust entries (no decay).
4. Metrics: `memory_entries_expired_total`.

## Other information

- Design context: `docs/design/MEMORY.md`, `docs/design/SECURITY.md`.

- [ ] This might be a breaking change

Leggi prima docs/design/MEMORY.md e docs/design/SECURITY.md, quindi traccia i punti di ingresso per la scrittura e il recupero della memoria di Agent (Python runtime). Definisci come si integrano i TTLs per voce, il decadimento basato sulla source, la scadenza lazy o in background, il pinning e la metrica memory_entries_expired_total; il lavoro è completo quando il design copre ogni comportamento proposto e il relativo impatto di breaking change.

Scritto dal modello di indicizzazione a partire dal testo della issue.