Fix qmax-code installer guidance

[Desperado]

Summary

• remove stale qmax CLI install/login guidance from the qmax-code installer
• show the installed qmax-code version and release URL after install
• point users at standalone qmax-code setup commands and auth options
• update README install/intro text to match the standalone API flow

Tests

• bash -n install.sh
• go test ./...
• git diff --check

[qualitymaxapp]

✅ QualityMax Pipeline

Gate	Result
🔍 AI Review	✅ Clean
🧪 Repo Tests	✅ 407/407 passed (go)
🤖 AI Tests	✅ 28/32 passed

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