SchematicCommandHandler:
- Add validation for schematic names to prevent path traversal
- Restrict names to alphanumeric, hyphens, and underscores only
- Add length limit (max 64 characters)
- Apply validation to both save and paste commands
- Prevent filesystem attacks via malicious schematic names
Security improvement: blocks names like "../../config" or "../../../etc/passwd"
All changes tested and verified with build + tests passing.
