Designer and builder: prompts, skills and DESIGN.md for concepts

Imported from GitHub issue El-SaMa/tekori#11 by @El-SaMa.

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Tasks

• DESIGN.md gains: Mode:, Concept: (name + one line why), ## Composition (the concept applied to this page: storyboard of first screen and 3-4 scroll positions), ## Still version (what reduced-motion users see).
• Designer step text in packages/agents/src/builder.ts per mode: safe as today; bold/experimental: pick one of the 3 candidate concepts (or justify another), map every checklist item into the composition, keep the main action on the first screen.
• Skills: composition.md gets "Breaking the grid well" (overlap with intent, one focal point per screen, reading order, white space as structure); motion.md gets pacing for scroll-driven pages (one moving idea per screen, scrub vs trigger, no scroll-jacking of the whole page); brand-system.md gets how a concept inherits the brand (colour roles over imagery, type scale for giant type).
• Builder: may not drop checklist items; may not add new boxed sections in experimental mode unless the concept says so.
• Editor role (later edits by chat) keeps the concept: an edit request that would break it gets a short note back to the user.

Acceptance criteria

• Snapshot tests of the first messages per mode.
• A dry run on nano/local shows the designer naming a concept and a storyboard (mechanics only; quality judged later in #16).

Depends on

#1-#4.

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Plan key T11. Epic: El-SaMa/tekori#19.