[7.9] Add _admin/ to .gitignore

[vbonk]

What needs to be done

Add to .gitignore: _admin/ directory. Must be done BEFORE creating any _admin/ files.

Acceptance Criteria

• _admin/ in .gitignore
• No _admin/ files tracked by git

[vbonk]

Completed in commit ae507aa (.claude session files + _admin/ added to .gitignore).