Slack MCP Server Authentication issue

## Issue Summary

The Slack MCP server provided by the `productivity` plugin fails to authenticate with error: **"Incompatible auth server: does not support dynamic client registration"**

## Error Details

When attempting to authenticate with the Slack MCP server through the Claude Code, the following error occurs:

```
Plugin: productivity:slack MCP Server
Status: ✘ failed
Auth: ✘ not authenticated
URL: https://mcp.slack.com/mcp
Config location: Dynamically configured

Error: Incompatible auth server: does not support dynamic client registration
```

## Configuration

### Current Slack MCP Configuration
```json
{
"mcpServers": {
"slack": {
"type": "http",
"url": "https://mcp.slack.com/mcp"
}
}
}
```

## Root Cause

Slack's OAuth server does not support **Dynamic Client Registration (DCR)**, which Claude Code appears to attempt by default for HTTP-based MCP servers. Slack requires pre-registered OAuth applications with explicit client credentials.

## Expected Behavior

One of the following should occur:

1. **Plugin pre-configuration:** The productivity plugin should either:
- Include pre-registered OAuth credentials for Slack
- Detect DCR incompatibility and guide users through manual OAuth setup
- Provide clear documentation on setting up OAuth credentials

2. **Graceful fallback:** Claude Code should detect when DCR is not supported and:
- Prompt the user to register an OAuth app
- Provide step-by-step instructions
- Offer to add the credentials via CLI

3. **Better error messaging:** The error should explain:
- Why authentication failed
- What the user needs to do
- Link to documentation or setup guide