Preserve symlinks and verify zip backups

[vosslab]

Title: Preserve symlinks and avoid large tmp data in account backups

Problem:

• Backups could dereference symlinks, storing link targets as regular files.
• Restores could prompt or fail when extracting over existing paths.
• The backup archive included .codex/tmp, which can be very large (100+ MB) while the rest of the account data is small.

Change:

• Use cp -RP and zip -y so symlinks are stored as symlinks.
• Remove ${tmpdir}/.codex/tmp before zipping to avoid saving transient patch/tmp data.
• Remove any existing archive before zipping to prevent stale entries.
• Extract into an empty subdirectory and use unzip -o to avoid interactive prompts and handle path edge cases.

User impact:

• Backups preserve symlinks and are much smaller (tmp excluded).
• Restores are non-interactive and more reliable.

Testing:

• Verified a symlink inside CODEX_HOME remains a symlink after save and restore.
• Verified .codex/tmp is not present in the archive and archive size drops substantially when tmp is large.
• Restored over an existing account without prompts.