feat: update Slack plugin to HTTP transport with OAuth config

- Switch from SSE to HTTP transport
- Update URL from /sse to /mcp endpoint
- Add OAuth clientId and callbackPort for Claude Code auth flow

.claude-plugin/marketplace.json

@@ -639,6 +639,16 @@
         "url": "https://github.com/sonatype/sonatype-guide-claude-plugin.git"
       },
       "homepage": "https://github.com/sonatype/sonatype-guide-claude-plugin.git"
+    },
+    {
+      "name": "firecrawl",
+      "description": "Web scraping and crawling powered by Firecrawl. Turn any website into clean, LLM-ready markdown or structured data. Scrape single pages, crawl entire sites, search the web, and extract structured information. Includes an AI agent for autonomous multi-source data gathering - just describe what you need and it finds, navigates, and extracts automatically.",
+      "category": "development",
+      "source": {
+        "source": "url",
+        "url": "https://github.com/firecrawl/firecrawl-claude-plugin.git"
+      },
+      "homepage": "https://github.com/firecrawl/firecrawl-claude-plugin.git"
     }
   ]
 }

external_plugins/slack/.mcp.json

@@ -1,6 +1,10 @@
 {
   "slack": {
-    "type": "sse",
-    "url": "https://mcp.slack.com/sse"
+    "type": "http",
+    "url": "https://mcp.slack.com/mcp",
+    "oauth": {
+      "clientId": "1601185624273.8899143856786",
+      "callbackPort": 3118
+    }
   }
 }