Fixes #3975. Add specs/constitution.md to consolidate tenets and engineering philosophy

[Copilot]

Terminal.Gui's design tenets were scattered across six files with no single authoritative source. This adds specs/constitution.md as that source, and updates CONTRIBUTING.md and CLAUDE.md to point to it.

Changes

• specs/constitution.md (new) — Authoritative constitution with:

  • Mission statement and explicit non-goals
  • Design tenets in precedence order, consolidating from keyboard.md, mouse.md, and newinv2.md (Users Have Final Control → Keyboard First → More Editor Than CLI → Platform Consistency → If It's Hot It Works → Separation of Concerns → Testability → Performance → Docs are the Spec → Graphemes Not Runes)
  • Engineering philosophy (Amazon PE Community Tenets)
  • Code style tenets (sourced from CONTRIBUTING.md)
  • Sub-project relationship clause

• CONTRIBUTING.md — Header blockquote now references specs/constitution.md

• CLAUDE.md — Header blockquote now references specs/constitution.md for AI agents

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working