Rework headless documentation: remove 'for dummies' and tutorial files, consolidate into integrated docs
- Deleted Headless-mode-for-dummies.md (outdated GAMA 1.7 tutorial)
- Deleted HeadlessTutorial.md (redundant, Windows-specific code)
- Rewrote RunningHeadless.md as unified overview (3 modes, bash wrapper, Docker, calling from other languages)
- Redefined HeadlessLegacy.md (XML-based, concise XML reference, output structure, common errors)
- Redefined HeadlessBatch.md (batch experiments defined in GAML, clean example)
- Cleaned HeadlessServer.md (removed old Python/JS code snippets, added protocol overview, message types, client library references)
- Updated _Sidebar.md (reordered Running Headless section, removed deleted file references)
Phase 7: Migration guide, GAML cross-links, extension references
- Add Migrating-from-2025-06.md with compact migration guide (22 categories, full checklists)
- Update WikiOnly/GamlLanguage.md with cross-links for 20+ new keywords
- Add References/GAMLReferences/BuiltInFIPA.md — FIPA skill reference
- Add References/ExtensionStats.md — Stats operators (75+ operators documented)
- Add References/ExtensionSound.md — Sound extension reference
- Add References/ExtensionBDI.md — BDI operators and statements reference
- Update _Sidebar.md — migration guide and extension docs added