Add arbiter agent to gate memory retrieval interruptions

[Copilot]

Memory retrieval currently interrupts conversations (via shadow turns) automatically whenever new memories arrive, leading to poorly-timed or excessive interruptions. This adds a lightweight MemoryArbiter component that decides allow/deny/defer before any interruption fires.

New: src/memory/arbiter.ts

Stateless decision engine with three ordered policies:

1. Priority-tag force-allow — rationale containing any configured tag (e.g. "safety", "billing") bypasses all other rules
2. Rate-limit — denies if last allowed interruption was within rateLimitMs (default 30 s); returns suggestedDelayMs
3. Confidence threshold — denies if score below confidenceThreshold (default 0.5); score = min(1, newItemCount / 3)

arbiter.decide({ query, newItemCount: 2, rationale: 'billing issue' })
// → { decision: 'allow', confidence: 1.0, reason: 'priority tag "billing" matched' }

Config: MemoryConfig (persisted in memory-config.json, tunable via /memory-config API)

Field	Default	Description
arbiter_enabled	false	Feature flag — off by default
arbiter_rate_limit_ms	30000	Min ms between allowed interruptions
arbiter_confidence_threshold	0.5	Min confidence to allow
arbiter_priority_tags	["safety","billing","urgent","critical"]	Force-allow tags

Integration: MemoryModule.retrieveWithLate()

• Syncs arbiter config on every call
• Gates both the sync newMemories return value and the async onLateArrival callback
• Suppressed decisions logged to console and persisted as memory_arbiter DB events (metadata only — no memory content in logs)
• Arbiter state resets on conversation complete and clearCache()

Tests: tests/unit/memory-arbiter.test.ts

10 unit tests covering allow, deny, rate-limit, priority-tag override, case-insensitive tag matching, zero items, reset, live reconfigure, and rapid-call suppression. Runnable via npm run test:arbiter.