Correct verified QNAP login guidance

[raclo]

Summary

• correct the QNAP automatic Bash-entry guard to detect the active Bash path instead of assuming BASH_VERSION is unset
• record the verified post-reboot QNAP/Entware test and the latest green main CI run
• document the safe $USER prompt fallback when Bash cannot expand \u
• ignore the local machine-specific QNAP_PRIVATE.md runbook without committing its contents

Why

Physical QNAP testing showed that QTS /bin/sh is its Bash 3.2 build running in sh mode. It still sets BASH_VERSION, so the original -z "${BASH_VERSION:-}" guard did not enter Entware Bash. Comparing $BASH with /opt/bin/bash handles that environment and prevents recursion.

Validation

• interactive SSH entered Entware Bash 5.3.9 automatically with SHLVL=1
• hmm loaded as a function and retained working search, pagination, and editable recall
• non-interactive ssh -T remained in QTS sh and terminated normally
• Entware, /opt, Bash, the corrected prompt, and hmm remained available after a full QTS reboot
• GitHub Actions run 30546897359 passed all 13 jobs on the merged main commit
• git diff --check passed and git check-ignore confirms QNAP_PRIVATE.md is excluded