feat(pattern-model): outcome-framed patterns + shared layer + web-product reframe

[efordee24]

Adopts the outcome-framed pattern model in kuat-agent-rules — implementing the resolved design in docs/migration/kuat-pattern-model.md (§9, Ed) per the execution prompt docs/migration/pattern-model-rollout-prompt.md. First cut: web-product full reframe + placeholder/shared scaffolding for slides & web-marketing. Blocks deferred.

Why

Patterns were UI-named, single-medium-only scenarios with no way to express a concept shared across media (e.g. a case study told in slides vs on a marketing page) without duplication. This adopts GOV.UK-style outcome framing (Ask users for… / Help users to… / Pages) and a concept↔implementation split: shared concepts factored to a top-level layer only when cross-medium, single-medium patterns staying in-medium, and a medium's patterns/ folder only ever holding patterns it implements (no bloat).

What changed

• Gate — extended skills/scripts/check-reference.mjs structure() to allow the shared layer reference/patterns/ alongside media-types/<medium>/patterns/. (Required first: the check walks the whole tree, so the new folder was illegal until extended.)
• Shared layer — reference/patterns/overview.md (the model, passive) + help-users/describe-a-case-study.md (the worked cross-medium concept — slides implementation linked, web-marketing TBD, no web-product entry).
• web-product reframe + rename (git mv, history preserved): authentication→sign-in, forms→complete-a-form, dashboards→dashboard, documentation kept. Each leads user goal → context → principles, folds existing layout/design/content/a11y/impl detail under Solution in web-product, then Best Practices + Examples. README reindexed by Ask/Help/Pages.
• slides + web-marketing — light-touch passive "pattern model (later cut)" notes; slides/patterns/case-studies.md gains an Implements: link to the shared concept.
• Contribution — new contribute/proposing-a-pattern.md (Medium/Heavy; proposing-a-block deferred), linked from overview, CONTRIBUTING, and proposing-a-reference-change.
• Consumption — create-web-app pattern-loading refs updated to the reframed slugs; renames recorded in reference/MIGRATION-MAP.md.
• Payloads — plugins/** regenerated via build:plugins.

Reviewer notes

• Blocks are deliberately deferred — no pattern↔Block registry, no proposing-a-block; docs only mention a Block could link later.
• plugins/** changes are generated (npm run build:plugins) — review the source under reference/, contribute/, skills/; do not hand-edit payloads.
• Historical docs/migration/ verification reports + eval outputs still cite the old pattern paths by design — they're point-in-time records; the rename is carried by MIGRATION-MAP, not by rewriting history.

Verification (all green)

• reference:check --all — passive, 337 links resolve, structure, token drift ✅
• verify:plugins — both payloads, snapshot 0 broken, 7 contributor skills kept out ✅
• no-bloat grep — no case-study file under web-product; each medium's patterns/ holds only its own ✅

Full write-up: docs/migration/report-pattern-model.md. Decisions logged in docs/migration/LOG.md.

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