Rename: update internal gui-cs → tui-cs references

[tig]

Tracking issue for the org rename gui-cs → tui-cs as it applies to this repo.

GitHub auto-redirects all github.com/gui-cs/* URLs; this updates internal references for self-consistency.

Rule

• Every gui-cs (hyphen, the org login) → tui-cs
• Historical brand gui.cs (dot) left as-is
• Machine-local absolute paths left untouched

Sequencing

Prep-first: PR staged as draft now, merged after the org rename.

PR linked below.