[Suggestion]: support scan_inclusions

### Which fff frontend(s)?

Core or Rust crate

### What problem are you trying to solve?

An option to provide scan_inclusions, namely to enable explicit inclusions of otherwise ignored paths.

This would I think be a better solve than #108/#530 (request for no-gitignore)
This basically would close #80
This is similar to #448 but it scopes the list up instead of down

The argument for scan_inclusions is that increasingly repos need to control what their agents can find, and also what can be fuzzy-found from file search UIs. It would be generally useful, and as a few examples of common cases of gitignored files that many repos would like to make easily findable: .env.local, log files, skills localed in home dir.

prior art here is zed, which supports exactly this through file_scan_inclusions setting.

I have a PR (claude) but I didnt want to open it without discussion since IANA rust dev

### Proposed solution

scan_inclusions as a list of glob patterns relative to the indexed root

https://github.com/rt2zz/fff/commit/f33677a0946d0efa831e0a978968e092974f7cba

Start by reviewing the Rust crate's existing scan and ignore configuration, then inspect the referenced commit f33677a0946d0efa831e0a978968e092974f7cba. Confirm how glob patterns relative to the indexed root should interact with ignored paths; done means scan_inclusions explicitly makes matching ignored paths findable without changing unrelated exclusions.

Written by the indexing model from the issue text.