Add Slack Block Kit rendering to the C# weather + WorkIQ agent

[vinchencc]

What

Renders the C# Agent Framework weather + WorkIQ agent's responses as native Slack Block Kit cards when the channel is Slack. Other channels (Teams, etc.) keep the existing streaming text experience.

How

• Added Microsoft.Agents.Extensions.Slack; marked WeatherAgent [SlackExtension] / partial.
• New PostSlackBlocksAsync: when Activity.ChannelId == "slack", collects the full response and posts a Block Kit card via SlackExtension.CallAsync (chat.postMessage).
• Card: blue accent bar (#2E9BF5) + header + mrkdwn section + divider + context footer.
• .gitignore: a365 CLI config, generated appPackage, and the Slack per-conversation state folder (contains API tokens).

Validation (live in Slack via Azure Bot Service)

• Weather → forecast card.
• WorkIQ → ListTeams ("Agents SDK 4") and list-channels ("General") returned real tenant data, rendered as cards (OBO sign-in).

Follow-ups

• Perf: cache WorkIQ MCP tools instead of reloading per turn (~5s latency).
• Interactive Slack actions (/api/actions) not included (outbound Block rendering only).

[vinchencc]

Live in Slack — weather + WorkIQ (ListTeams / list-channels) rendered as Block Kit cards.

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