[WIP] Add message tool integration for Discord

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Implement message tool (Discord/Matrix)</issue_title>
<issue_description>Priority: P1 (required for Rook autonomy)

Spec:

• Accept parameters: {channel, content, target?}
• Start with Discord webhook (simplest integration)
• Abstract via channel config in daemon.yaml (webhook URLs per channel name)
• Support basic message sending (no attachments/embeds initially)

Files:

• Create src/tools/message.ts
• Register in tool registry
• Add JSON Schema definition
• Extend config schema for messaging.channels

Tests:

• Mock webhook POST requests
• Integration test with agent loop
• Error handling (invalid webhook, network failure)

Acceptance:

• Tool executes via agent loop
• Rook can post to Discord channels autonomously
• Config-driven channel selection

Future: Matrix support, rich embeds, attachments

Ref: gap-analysis-report.md P1-4</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Implement message tool (Discord/Matrix) #125

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