Add Sigment.dev to llms.txt directory

Site Name: Sigment
URL: https://sigment.dev/
llms.txt Path: https://sigment.dev/llms.txt
llms-full.txt Path: https://sigment.dev/llms-full.txt

Description:
Sigment is a lightweight, HTML-first JavaScript framework designed for fine-grained reactivity. It supports both SPA and Islands Architecture patterns. We have implemented the llms.txt standard to ensure our framework's rules and architecture are natively discoverable and interpretable by AI agents and coding assistants.

I am the creator of Sigment and would love to have it included in the community list of sites adopting this standard!

Check the repository's existing directory entries and contribution format first. Add Sigment using the supplied site URL, /llms.txt URL, and /llms-full.txt URL, then verify both links resolve and the entry matches neighboring records.

Written by the indexing model from the issue text.