fix(mcp): support custom Chrome user data dirs

[yize]

What changed

• add --user-data-dir <path> and --user-data-dir=<path> support to the native-host installer
• add XSWITCH_MCP_USER_DATA_DIR for automated installs while preserving XSWITCH_MCP_MANIFEST_PATH as the highest-priority exact override
• document that manifests belong under <user-data-dir>/NativeMessagingHosts/, not inside an individual Chrome profile
• add install, uninstall, environment precedence, path-with-spaces, and validation coverage
• update xswitch-mcp to version 0.3.1

Why

Chrome instances launched with a custom --user-data-dir resolve per-user Native Messaging manifests from that data root. The installer previously always wrote to the default Chrome data directory, causing Specified native messaging host not found even though the runtime was installed correctly.

Validation

• npx eslint mcp/install-host.js __tests__/mcp-installer.spec.ts
• npm test — 34 tests passed
• npm run build
• npm --prefix mcp run build
• packed xswitch-mcp-0.3.1.tgz and verified custom-directory install/uninstall end to end