Add generic compaction item contract

[chubes4]

Problem

AgentConversationCompaction is currently transcript-shaped: it operates on normalized message envelopes and uses conversation-specific boundary rules. Persistent memory compaction needs the same safety model, but over generic ordered items such as markdown sections, memory records, daily archive chunks, or tool traces.

Goal

Introduce a generic compaction item contract that conversation compaction and future memory compaction can both use without forcing markdown sections into fake chat messages.

Proposed Shape

• Define a normalized item shape or value object with:
  • id
  • type
  • content
  • metadata
  • optional grouping/boundary hints
• Add normalization helpers similar to AgentMessageEnvelope::normalize_many().
• Keep provider/model execution out of Agents API.
• Keep Data Machine vocabulary out of Agents API.

Acceptance Criteria

• Agents API exposes a public item normalization contract for compaction inputs.
• Conversation compaction can continue using message envelopes without breaking public API.
• Tests cover valid items, invalid/missing item fields, stable IDs, metadata preservation, and ordering.

Notes

This is the foundation for later MEMORY.md and daily-memory compaction without coupling Agents API to Data Machine files.