feat(memory): add expiry, supersession, and TTL support

[Siddhant-K-code]

What

Adds the ability to mark memory entries as expired or superseded, with TTL support on write.

Why

Memory that can't be pruned becomes noise. When a decision is reversed or information becomes outdated, there was no way to mark it — stale entries degraded retrieval quality over time.

Changes

Core (pkg/memory)

• Expire() — marks entries as expired; they remain in the store but are excluded from Recall by default
• Supersede() — marks an entry as expired and stores a forward pointer to its replacement
• StoreEntry.ExpiresAt — optional TTL; entries past their expiry time are filtered at query time
• RecallRequest.IncludeExpired — opt-in flag to retrieve expired entries
• Stats now reports expired_count and active_count
• Dedup skips expired entries so replacements aren't silently merged
• EventExpired lifecycle event for cache boundary managers

API

• POST /v1/memory/expire — expire entries by ID
• POST /v1/memory/supersede — supersede one entry with another

MCP

• memory_expire tool
• memory_supersede tool

Tests

• 12 new tests: expire, double-expire, supersede, supersede-not-found, supersede-already-expired, lifecycle events, TTL past/future, stats with expired count, dedup-skips-expired, empty request, empty old_id

Usage

# Expire an entry
curl -X POST localhost:8080/v1/memory/expire \
  -d '{"ids": ["abc123"]}'

# Supersede: old decision replaced by new one
curl -X POST localhost:8080/v1/memory/supersede \
  -d '{"old_id": "abc123", "new_id": "def456"}'

# Store with TTL (auto-expires after 24h)
curl -X POST localhost:8080/v1/memory/store \
  -d '{"entries": [{"text": "temp context", "expires_at": "2026-05-10T00:00:00Z"}]}'

# Recall includes expired entries when needed
curl -X POST localhost:8080/v1/memory/recall \
  -d '{"query": "old decisions", "include_expired": true}'

Closes #79