Add mcp-context-warning plugin to warn about high MCP context usage

This plugin warns users when their MCP servers are consuming significant
context (>20k tokens). Many users don't realize MCP servers take up context
space, which can lead to unexpectedly fast context exhaustion.

The plugin:
- Runs a SessionStart hook to check MCP configurations
- Estimates token usage based on server and tool counts
- Shows a warning with server breakdown if estimated usage > 20k tokens
- Provides guidance on how to disable unused MCP servers

Slack thread: https://anthropic.slack.com/archives/C07VBSHV7EV/p1765989316572149

plugins/mcp-context-warning/.claude-plugin/plugin.json

@@ -0,0 +1,9 @@
+{
+  "name": "mcp-context-warning",
+  "version": "1.0.0",
+  "description": "Warns users when MCP servers are consuming significant context (>20k tokens), helping prevent unexpected context exhaustion",
+  "author": {
+    "name": "Anthropic",
+    "email": "support@anthropic.com"
+  }
+}