docs: update engine support baseline to Godot 4.6+ and document desktop GDExtension behavior

[gumaciel]

Summary

• Updates the documented minimum engine support baseline to Godot 4.6+ (which introduced the modern godot_apple_embedded architecture and automated Swift toolchain search path handling).
• Documents include_tags = ["ios"], the expected non-fatal notice in Godot 4.3–4.7 desktop editor runs, and upstream Godot 4.8+ tag evaluation (godotengine/godot#121575).
• Updates AGENTS.md with current file map and engine baseline.

Verification

• swift test passed with 8/8 tests passing.

[poing-ai]

Poing AI

Commit: f68b71ea8d · Model: gemini-3.8-flash

✅ Approved

Updates documentation and repository guidelines to specify Godot 4.6+ as the engine baseline and documents GDExtension configuration and desktop loading behavior across Godot versions.

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