Mailbox: extend memory-core healthcheck with inbox + outbox preview

Context

Parent #10139 phase 3. Healthcheck is called on every agent boot (AGENTS_STARTUP.md §6); piggybacking mail-preview is a free notification channel — zero new polling loop.

The Problem

Session start has no "previous-you (or anyone) left a message" signal. Agents discover stale handoffs through manual query or don't discover them. Amnesia boundary = guaranteed rediscovery cost per session.

The Architectural Reality

• mcp__neo-mjs-memory-core__healthcheck returns structured JSON today; extension is additive.
• Caller-identity resolved post-OAuth2; inbox scoping is a simple edge query.
• Preview cap: 3 items inbox + 3 items outbox — tight ceiling.

The Fix

• Extend payload with mailbox: {unreadCount: N, inbox: [...3], outboxRecent: [...3]}
• Each preview item: {id, from, subject, createdAt, priority?} — subject truncated to ~60 chars
• Query: two indexed SELECT against Message nodes with caller-identity scope; no FTS, no Chroma, no graph traversal beyond SENT_BY / SENT_TO lookups
• Sorted by createdAt DESC; filters out archived + retracted
• Zero-cost when empty

Acceptance Criteria

• Healthcheck payload includes mailbox field
• Inbox preview: up to 3 unread, most recent first
• Outbox preview: up to 3 most recent sent (regardless of receiver state)
• Truncated subject + stable item shape documented
• Performance observability: healthcheck latency instrumented; no p95 regression vs baseline
• Archived/retracted excluded from inbox preview

Out of Scope

• Per-message priority-sorting (recency default; priority-sort if friction surfaces)
• Full-text search in preview (use listMessages or semantic layer)
• Healthcheck pagination (not needed at 3+3 cap)

Avoided Traps

• Full bodies in preview. Rejected. Healthcheck is hot path; preview signals, doesn't read.
• Preview count > 5. Rejected. Ceiling keeps healthcheck cheap — every agent polls on every boot.

Related

• Parent: #10139
• Depends on: Message schema sub, #10016 OAuth2 (identity scoping)
• Files: ai/mcp/server/memory-core/ healthcheck handler

Origin Session ID: 71dc3cd8-d39d-48e1-ac62-e240ca67d1a5