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/hooks/hooks.json

@@ -0,0 +1,17 @@
+{
+  "description": "Warns users when MCP servers consume significant context (>20k tokens)",
+  "hooks": {
+    "SessionStart": [
+      {
+        "matcher": "*",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "python3 ${CLAUDE_PLUGIN_ROOT}/hooks/check_mcp_context.py",
+            "timeout": 10
+          }
+        ]
+      }
+    ]
+  }
+}