fix: use relative path in .mcp.json to avoid CLAUDE_PLUGIN_ROOT warning

${CLAUDE_PLUGIN_ROOT} is only defined when the repo is loaded as an installed
Claude Code plugin. When contributors open the repo as a regular project, the
variable is undefined and Claude Code prints:

  [Warning] mcpServers.context-mode: Missing environment variables: CLAUDE_PLUGIN_ROOT

Use a relative path so the MCP entry works for repo-as-project usage. Plugin
install path is unaffected — it still uses .claude-plugin/plugin.json.

Closes #251

Co-authored-by: Ercan Ermis <eposta@ercanermis.com>

.mcp.json

@@ -2,7 +2,7 @@
   "mcpServers": {
     "context-mode": {
       "command": "node",
-      "args": ["${CLAUDE_PLUGIN_ROOT}/start.mjs"]
+      "args": ["./start.mjs"]
     }
   }
 }