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 2 potential issues.

View 5 additional findings in Devin Review.

[devin-ai-integration]

🔴 is_human_sender falls through to heuristic for SenderKind::System, allowing potential ACL bypass

The is_human_sender function in src/control.rs only explicitly handles SenderKind::Human (return true) and SenderKind::Agent (return false). The new SenderKind::System variant falls through to the string-based heuristic, which checks whether the sender name starts with "human:". If a system-kind sender happens to have a name matching the human heuristic (e.g. "human:scheduler"), is_human_sender would incorrectly return true. This matters because can_release_child at src/control.rs:16 grants release privileges to human senders — so a misclassified system sender could bypass the owner-check ACL.

(Refers to lines 8-13)

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[devin-ai-integration]

🔴 Python Message.from_kind is a required field without a default, breaking the public API

In packages/sdk-py/src/agent_relay/relay.py, the Message dataclass has from_kind: Optional[SenderKind] inserted between from_name and to without a default value of None. Despite being Optional, Python dataclasses require fields without defaults to be provided by all callers. Since Message is exported in __all__ (packages/sdk-py/src/agent_relay/__init__.py:82), any external code constructing a Message without specifying from_kind will get a TypeError. Adding = None would maintain backward compatibility.

Suggested change

	from_kind: Optional[SenderKind]
	from_kind: Optional[SenderKind] = None

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