Make building widgets a documented, contributable surface.

Part of the Modular Dashboard roadmap — see the tracker #101136. Builds on Wave 1 (#101093; PRs #101094 / #101097 / #101098).

Problem

Adding a builtin widget or a custom widget today has no documented pattern or starter — contributors and agents have to reverse-engineer an existing renderer each time. That's fine for the Wave 1 authors, but it doesn't scale to a community of contributors building widgets independently.

What this adds

• A documented builtin contract: what a builtin renderer receives and must return.
• A template renderX(widget, value, ctx) => TemplateResult starter for new builtins.
• Custom-widget HTML scaffolds: notes, calculator, chess.
• A short authoring guide tying the two paths (builtin vs. custom) together.

How it'd be built

Document the contract implied by BUILTIN_WIDGET_RENDERERS and getBuiltinRenderer in ui/src/lib/dashboard/widgets/index.ts, using renderStatCard in ui/src/lib/dashboard/widgets/stat-card.ts as the representative example. Adding a new builtin kind also means extending BUILTIN_KIND_PATTERN in extensions/dashboard/src/schema.ts — the guide should call out that both places need to change together, since that's the easiest step to miss.

References

• [Feature]: Make OpenClaw Feel Like WordPress: A UX Revolution for Effortless Configuration #66138 — WordPress-like widget ask; this is the prerequisite documentation layer.
• [RFC] Control UI plugin contribution slots #71736 — related widget authoring/discovery ask.

Tier 0 — multipliers.