docs(ai-primer): document four high-value v2 gotchas agents miss

[tig]

First, safe slice of #5525. Adds the verified, high-frequency gotchas to ai-v2-primer.md's Gotchas → API Correctness list — the ones agents get wrong even once they know it's v2, and which the primer didn't cover:

1. Dialog/MessageBox button order = the default — last button added is the default; order Cancel … OK; don't hand-set IsDefault. (Views/Dialog.cs, Views/MessageBox.cs)
2. AddCommand is protected — register inside the View, bind via KeyBindings. (ViewBase/View.Command.cs)
3. Terminal.Gui.Drawing.Attribute collides with System.Attribute. (Drawing/Attribute.cs)
4. Typed views expose .Value via IValue<T>, not .Date/.Color. (Views/DatePicker.cs)

All inline prose (no fenced snippets), so validate-doc-snippets is unaffected. Each verified against current develop source.

Remaining #5525 items — flagged for a maintainer call (not in this PR)

These are contentious or cross-file, so they want your decision rather than a unilateral edit:

• Accepted vs Accepting: the primer says "Accepted for side effects, Accepting to cancel," but live UICatalog scenarios use Accepting for side effects ~139:10, and getting-started.md says event handling "uses Accepting." Pick one and make the docs + examples agree (then validate-doc-snippets can enforce it).
• Cookbook / llms.txt model manual IsDefault = true and obsolete SelectedItemChanged — update to match gotchas There is a problem with the high-intensity colors, they are not showing up #1/Add mouse event support to the core. #4 above.
• Unify version/maturity strings (@2.0.* vs alpha.* vs beta.*; "Alpha" vs "Beta") now that v2 is stable 2.4.x.
• Add llms.txt (and the dialog/IValue snippets) to validate-doc-snippets.yml so they can't rot.

Sourced while building the agent-optimized templates (tui-cs/Terminal.Gui.templates#22).