fix(install): .mcp.json source template uses ${CLAUDE_PLUGIN_ROOT}/start.mjs (closes #531 partial — 1 of 10)

The shipped .mcp.json template at repo root used a bare relative
'./start.mjs' arg since commit aea633c (#253). Claude Code spawns
the MCP child with session CWD inherited, not pluginRoot — so
fresh npm marketplace installs (which never trigger cli.ts
upgrade()) ship the broken file and Claude Code throws
MODULE_NOT_FOUND on every ctx_* tool.

This was the asymmetric-heal sibling we missed in v1.0.119 #523:
healPluginJsonMcpServers only repaired .claude-plugin/plugin.json
mcpServers.args, never the sibling .mcp.json.

This is slice 1 of 10. Remaining slices: healMcpJsonArgs deep
module + wire into start.mjs/postinstall/cli.ts + CI invariant
that prevents the asymmetric-drift class permanently.

.mcp.json

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