🛡️ Sentinel: Fix insecure shell variable expansion in entrypoint

[google-labs-jules]

🛡️ Sentinel: [HIGH] Fix shell variable expansion in password handling

Severity: HIGH (Potential credential corruption)
Vulnerability: Passwords containing spaces were being split into multiple arguments when passed to adduser, causing the password to be truncated or the command to fail. Additionally, backslashes in credentials would be interpreted as escape characters.
Impact: Users attempting to set strong passwords with phrases or special characters would silently get incorrect passwords set, or fail to create users.
Fix:

• Quoted $username and $password when calling adduser.
• Added -r flag to read commands to disable backslash escaping.
  Verification: Added tests/verify_password_fix.sh which mocks the logic and asserts that passwords with spaces are preserved correctly.

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PR created automatically by Jules for task 8177214674040794402 started by @bluPhy

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