refactor(client)!: converge DM types on core message payload

[barryonthecape]

Summary

Per request, this PR is now client-breaking only (major-version style) and keeps server behavior unchanged.

Scope (clean)

Changed files are only:

• packages/types/src/message.ts
• packages/types/src/events.ts
• packages/types/src/dm.ts
• packages/sdk-rust/src/types.rs
• docs/message-model-convergence.md

No server route/engine behavior changes in this PR.

Breaking client changes

@relaycast/types

• Introduces shared CoreMessagePayloadSchema in message.ts:
  • id, agent_id?, agent_name, text, injection_mode?
• Event schemas converge on shared core payload (events.ts):
  • message.updated, thread.reply, dm.received, group_dm.received
• DM typed response models are now unified and breaking (dm.ts):
  • SendDmResponse => { conversation_id, message, created_at }
  • GroupDmMessageResponse => { conversation_id, message, created_at }

Rust SDK typed models

• DmSendResponse and GroupDmMessageResponse now use nested message core payload shape (breaking for typed consumers).

Why this is safe operationally

• Server can continue emitting legacy fields; client model change is the intentional major-version break.
• This lets us ship a clean, explicit client migration path now, then tighten server payloads later.

Validation

• packages/types: npm run build
• packages/sdk-rust: cargo test -p relaycast --tests

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[github-actions]

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[devin-ai-integration]

Devin Review found 1 new potential issue.

🐛 1 issue in files not directly in the diff

🐛 Python SDK FileAttachment model has stale fields, causing Pydantic validation errors for DM messages with attachments (packages/sdk-python/src/relay_sdk/models.py:132-136)

The Python SDK's FileAttachment model at packages/sdk-python/src/relay_sdk/models.py:132-136 expects url: str and size: int, but the server returns content_type and size_bytes for all attachments (see packages/server/src/engine/dm.ts:39-43 and packages/server/src/engine/message.ts:28-32). This PR introduces attachment support for DMs, so when dms.messages() fetches a DM conversation containing attachments, MessageWithMeta.model_validate(m) at packages/sdk-python/src/relay_sdk/agent.py:67 will raise a Pydantic ValidationError because each attachment dict has content_type/size_bytes instead of the expected url/size fields.

View 17 additional findings in Devin Review.