Route system-channel outbound messages to origin channel and add test

[YspCoder]

Motivation

• System-originated inbound messages encoded an origin in ChatID but outbound messages continued to use the system channel, preventing replies from being delivered back to the originating channel.

Description

• Update prepareOutbound to remap outbound Channel and ChatID when an inbound message has Channel == "system" by splitting ChatID on ":" and falling back to cli when no origin is present.
• Construct an outbound variable to modify the message before returning, trimming ReplyToID and preserving original content and chat info.
• Add a new unit test TestProcessInbound_SystemMessagePublishesToOriginChannel in loop_session_regression_test.go and import time for test timeout handling.

Testing

• Ran the package unit tests with go test ./pkg/agent, and the suite passed including the new TestProcessInbound_SystemMessagePublishesToOriginChannel test.

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Codex Task