refactor: extract JSON pointer operations from config.rs

[chubes4]

Problem

src/core/config.rs is 1,532 lines and serves as the central persistence layer. It contains several distinct concerns mixed together:

1. Config entity trait + macro — the entity_crud! macro and trait system
2. JSON pointer operations — get_pointer, set_pointer, delete_pointer, path resolution (~200 lines)
3. JSON parsing utilities — type detection, merging, diffing
4. File I/O — read/write JSON config files
5. Entity search — fuzzy matching, array field collection
6. Tests — ~300 lines

Proposed Change

Extract the JSON pointer operations into src/core/json_pointer.rs:

• get_pointer() / set_pointer() / delete_pointer()
• Path segment parsing and resolution
• Related helper functions

This is a self-contained, reusable piece of infrastructure that has no config-specific dependencies.

Benefits

• config.rs drops to ~1,300 lines (still large but more focused)
• JSON pointer ops become independently testable and reusable
• Clearer separation: config = entity persistence, json_pointer = generic JSON path operations

Constraints

• No behavioral changes
• All existing callers continue to work (re-export from config if needed during transition)