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

Read docs/design/MEMORY.md and docs/design/SECURITY.md first, then trace the Agent (Python runtime) memory write and retrieval entry points. Define how per-entry TTLs, source-based decay, lazy or background expiry, pinning, and the memory_entries_expired_total metric fit together; done means the design covers each proposed behavior and its breaking-change impact.

Written by the indexing model from the issue text.