Add harness workflow and Material showcase design system

docs/exec-plans/2026-03-23-harness-foundation.md

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+# Harness Foundation
+
+- Status: `completed`
+- Owner: `codex`
+- Date: `2026-03-23`
+
+## Goal
+
+Introduce the first harness-engineering layer for Cadence UI so the repository becomes easier for
+agents to understand, plan against, and validate mechanically.
+
+## Scope
+
+- In scope:
+  - document the harness workflow
+  - add execution-plan conventions
+  - expose shared validation suites from one entrypoint
+  - add a pull request workflow that runs the PR validation suite
+- Out of scope:
+  - full worktree orchestration
+  - change-aware suite selection
+  - fixing pre-existing lint and Storybook smoke issues discovered during baseline review
+
+## Constraints
+
+- Keep the existing package, registry, and Storybook workflows intact.
+- Avoid introducing a hard dependency on a specific coding-agent CLI.
+- Preserve the current release scripts while adding a richer harness path alongside them.
+
+## Affected Surfaces
+
+- `AGENTS.md`
+- `CONTRIBUTING.md`
+- `README.md`
+- `docs/harness-engineering.md`
+- `docs/exec-plans/*`
+- `package.json`
+- `.github/workflows/harness-validate.yml`
+- `scripts/harness/validate.mjs`
+
+## Plan
+
+1. Document what harness engineering means for this repository.
+2. Add versioned execution-plan guidance and a real example plan.
+3. Expose reusable validation suites behind a single script.
+4. Run the new PR suite from GitHub Actions.
+
+## Validation
+
+- `pnpm harness:suites`
+- `pnpm harness:validate:pr -- --dry-run`
+
+Baseline findings captured during rollout:
+
+- `pnpm test` passed
+- `pnpm typecheck` passed
+- `pnpm lint` failed on pre-existing story/test lint issues plus React Compiler hook lint
+- `pnpm test:e2e:smoke` reached a real Storybook drift failure in the button playground flow once
+  browser launch permissions were provided
+
+## Status Log
+
+- `2026-03-23 12:18` inspected repo structure, QA stack, CI workflows, and agent guidance
+- `2026-03-23 12:24` confirmed `test` and `typecheck` pass; found pre-existing `lint` failures
+- `2026-03-23 12:26` reran Playwright smoke outside the sandbox and confirmed one real Storybook
+  smoke failure instead of a harness-only environment failure
+- `2026-03-23 12:31` added harness docs, execution-plan conventions, validation script, and PR
+  workflow