Add gum-skills/ folder with 5 consumer-facing AI skills (#2700)

[vchelaru]

Closes #2700.

What

Adds a top-level gum-skills/ folder: external-facing AI skills a consumer drops into their project's .claude/skills/ to work with Gum. This is distinct from the engine-internal .claude/skills/ set (which is about working on Gum), and it fulfills the folder the published docs (docs/ai/ai-skills.md) already link to but which didn't exist yet.

gum-skills/
  README.md
  gum-overview/SKILL.md
  gumcli/SKILL.md
  gum-file-format/SKILL.md
  gum-forms-controls/SKILL.md
  gum-layout/SKILL.md

• gum-overview — what Gum is, project file types (.gumx/.gusx/.gucx/.gutx/.behx), Forms-vs-raw-visuals, the three usage modes, project wiring (GumService, Gum.MonoGame/Gum.KNI/Gum.FNA), and the content-copy rule (<None Update="Content\**\*.*" CopyToOutputDirectory="PreserveNewest">, not Content.mgcb) — item 7 from AI Dumping ground #2197.
• gumcli — recipe-oriented edit → validate → verify loop (codegen, check, screenshot).
• gum-file-format — the original AI Dumping ground #2197 ask: enum→int tables for WidthUnits/HeightUnits (DimensionUnitType), XUnits/YUnits (PositionUnitType), XOrigin/YOrigin (HorizontalAlignment/VerticalAlignment), and ChildrenLayout; qualified InstanceName.Property variables; instance parenting via the Parent variable; and the silent-failure landmines (enum-as-int, two on-disk variable shapes, silently-dropped element names that gumcli check catches).
• gum-forms-controls — controls vs raw visuals, composition via AddChild, and the state/category system brief (item 8 from AI Dumping ground #2197).
• gum-layout — units, anchor/dock, child stacking.

The README.md covers the 3rd-party-vs-internal distinction, a getting-started set, and a short "how to author a new skill" section.

Why these are more self-contained than the internal skills

These ship into a consumer's repo, so the reader has no Gum source tree or docs/ folder. The skills therefore state the concrete rules (enum tables, APIs) inline and link to the public docs site (docs.flatredball.com/gum/...) for depth, rather than pointing at repo source paths.

Verification (DoD #3/#4)

All factual content is grounded in source (enum values, GumService init, package IDs, and the Anchor/Dock/AddChild APIs), not from memory. Then a fresh sub-agent given only these skills attempted to build a small UI and hand-author a .gusx; its friction report surfaced a real blind spot around composition/parenting (adding a child control in code, nesting instances in XML, root-namespace boilerplate), which drove one round of fixes. The corrected file-format example now passes gumcli check and renders via gumcli screenshot.

A deeper test-run inside a full game project is left for follow-up as need is demonstrated — per the issue, this is deliberately not a long-running tracker; future skills (animation, hot reload, localization, styling, etc.) are each their own issue.

🤖 Generated with Claude Code