docs: add MIT license, explain reveal mode in README

Add LICENSE file (MIT). Explain in README that reveal mode is a
local display change only — the AI never received the real data.
Reveal exists so users can copy paths/commands from AI responses
and paste them directly into their terminal without manually
translating fake values back to real ones.

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

package.json

@@ -2,6 +2,7 @@
   "name": "silent-send",
   "version": "0.3.0",
   "private": true,
+  "license": "MIT",
   "description": "Browser extension that substitutes personal data before sending to AI services",
   "scripts": {
     "build:chrome": "./build.sh chrome",