Implement zero-friction authentication persistence with MCP user scope

Major breakthrough solving the authentication chicken-and-egg problem:

Key Changes:
- Copy ~/.claude.json and ~/.claude/ during Docker build for baked-in auth
- Add -s user flag to claude mcp add-json for persistent MCP servers
- Simplify rebuild logic to prevent unnecessary rebuilds
- Update documentation with rebuild instructions

Technical Details:
- Authentication files placed before USER switch in Dockerfile
- MCP configuration now persists across all sessions
- Rebuild only occurs when image doesn't exist
- Clean separation of build vs runtime concerns

Result: Users authenticate once on host, then zero login prompts forever.
SMS notifications ready immediately on container start.

.env.example

@@ -1,7 +1,7 @@
 # Copy this file to .env and fill in your credentials
 # The .env file will be baked into the Docker image during build
 
-# Required for Claude Code
+# Required for Claude Code if not using via subscription.
 ANTHROPIC_API_KEY=your_anthropic_api_key_here
 
 # Optional: Twilio credentials for SMS notifications