feat(discord): include channel name in dynamic context

[Alix-007]

Summary

• include the Discord channel name in inbound metadata when it is available from state or API lookup
• thread the optional chat name through agent process options into the dynamic system context
• add regression tests for Discord channel-name resolution and dynamic context rendering

Closes #1451.

Testing

• go test ./pkg/agent -run TestBuildMessages_IncludesChatNameInDynamicContext -count=1
• go test ./pkg/agent -run TestTargetReasoningChannelID_AllChannels -count=1
• go test ./pkg/channels/discord -run TestResolveChannelName_FromState -count=1

[Alix-007]

Closing this to keep the queue focused on higher-value bugfixes.

After reprioritizing the remaining open PRs, this one lands in the low-value bucket for now:

• the linked issue is labeled priority: low
• this is an enhancement rather than a bugfix
• the branch is now conflicting with main

If Discord context shaping becomes a higher-priority need later, it is better to reopen this as a fresh, narrower PR on top of current main.