Add FAQ guidance for OSStatus Error 80

[jamesyc]

Summary

• add an FAQ troubleshooting section for OSStatus Error 80 when reusing an existing Time Machine sparsebundle
• document checking for a stale sparsebundle lock file only after confirming the backup is not mounted or in use
• document removing matching sparsebundle Keychain entries, including System keychain entries, with security CLI examples

Validation

• reviewed the docs diff
• compared the suggested steps against public Time Machine OSStatus Error 80 reports and macOS security CLI documentation
• not run: test suite, docs-only change

[gemini-code-assist]

Code Review

This pull request adds a new troubleshooting section to the FAQ for resolving macOS OSStatus Error 80 during Time Machine backups. The feedback identifies a typo in a file extension and suggests improvements to the provided shell commands for better portability and correctness when managing keychain entries.