Implement slack.private.message tool for direct messaging

[fbraza]

Summary

• add SlackPrivateMessageParams with recipient name list and message text
• implement slack.private.message to resolve each name and send a direct message per user
• expand Slack prompt docs to explain the tool usage and parameters

Acceptance Criteria

• tool accepts 1..n human-provided names, resolves Slack users, and sends direct messages
• responses include success/failure details for each requested recipient
• automated tests cover happy path and partial failure scenarios

Notes

• reuse existing Slack WebClient from deps, preserve error handling patterns
• rely on slack.user.resolve tool for name → user_id resolution
• document expectations that group conversations are handled by a separate tool

Example

@slack_toolset.tool(name="slack.private.message")
def send_private_message(ctx: RunContext[Deps], params: SlackPrivateMessageParams) -> SlackDeliveryReport:
    matches = ctx.deps.tools.call("slack.user.resolve", {"names": params.users})
    results: list[DeliveryResult] = []

    for match in matches:
        try:
            response = ctx.deps.client.chat_postMessage(channel=match.user_id, text=params.text)
            results.append(DeliveryResult.success(requested=match.requested, user_id=match.user_id, ts=response["ts"]))
        except SlackApiError as exc:
            results.append(DeliveryResult.failure(requested=match.requested, reason=exc.response["error"]))

    return SlackDeliveryReport(results=results)