feat: update to cloud-hosted architecture with port 3045

Switch default port from 3000 to 3045 in Dockerfile and docker-compose.
Add MCP instructions to server for AI client guidance. Update README to
reflect cloud-hosted model with per-user headers, 42 tools, and session
management. Add .mcp.json to .gitignore to prevent leaking credentials.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -2,15 +2,15 @@ services:
   solidtime-mcp:
     build: .
     ports:
-      - "${PORT:-3000}:3000"
+      - "${PORT:-3045}:3045"
     environment:
-      - PORT=3000
+      - PORT=3045
       # Optional: default SolidTime API URL for all sessions.
       # Clients can override this via the x-solidtime-api-url header.
       - SOLIDTIME_API_URL=${SOLIDTIME_API_URL:-https://app.solidtime.io}
     restart: unless-stopped
     healthcheck:
-      test: ["CMD", "wget", "--no-verbose", "--tries=1", "--spider", "http://localhost:3000/health"]
+      test: ["CMD", "wget", "--no-verbose", "--tries=1", "--spider", "http://localhost:3045/health"]
       interval: 30s
       timeout: 5s
       retries: 3