feat: add first-class system handle semantics

[noodleonthecape]

Summary

• add first-class system-handle semantics across the Relay SDK and broker types
• model agent, human, and system participants separately instead of treating system as a special human
• update protocol/message mapping and tests to preserve sender kind semantics across the stack

Notes

This removes the old conflation where system() was effectively a human handle. System-origin messages remain ergonomic to send, but are now modeled as a distinct participant kind.

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[devin-ai-integration]

Devin Review found 1 new potential issue.

View 6 additional findings in Devin Review.

[devin-ai-integration]

🔴 Python SDK Agent.send_message does not pass from_kind to the broker client

In the Python SDK, Agent.send_message (packages/sdk-py/src/agent_relay/relay.py:204-212) does not pass from_kind to client.send_message(), even though the local Message object is constructed with from_kind="agent" on line 219. This is inconsistent with: (1) the TypeScript SDK's Agent.sendMessage which correctly passes fromKind: 'agent' at packages/sdk/src/relay.ts:1543, (2) the Python SDK's own _ParticipantHandle.send_message which correctly passes from_kind=self._kind at packages/sdk-py/src/agent_relay/relay.py:280. As a result, agent-originated messages from the Python SDK will not carry senderKind in the HTTP payload to the broker, meaning the broker cannot distinguish agent senders from unknown senders. The test at packages/sdk-py/tests/test_send_message_mode.py:89-93 expects from_kind="agent" to be passed and would fail.

(Refers to lines 204-213)

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