docs: add CONTRIBUTING.md, issue and PR templates

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Something doesn't behave the way it should
+labels: bug
+---
+
+## SSCCE
+
+A **Short, Self-Contained, Correct (Compilable) Example** — the smallest
+component usage that still reproduces the bug, with nothing to install or
+assume. Paste it as a single block; we should be able to copy it, run it,
+and see the bug with zero edits.
+
+```tsx
+// paste your minimal repro here
+```
+
+## Expected
+
+What you expected it to do.
+
+## Actual
+
+What it actually did.
+
+## Version
+
+- `@dunky-dev/*` package and version:

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true

.github/PULL_REQUEST_TEMPLATE.md

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+# Why/What
+
+<!-- Why this change, and what it does — a sentence or two. -->
+
+# Changes
+
+<!-- The change itself, in a few words. -->
+
+# Issues
+
+<!-- Link related issues, e.g. Closes #123 -->

CONTRIBUTING.md

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+# Contributing
+
+For the contribution _workflow_, code conventions, and boundaries, see [`AGENTS.md`](./AGENTS.md) — it's the same
+contract agents follow here, and it applies to you too.
+
+## Setup
+
+```bash
+git clone git@github.com:dunky-dev/ui.git
+cd ui
+nvm use
+corepack enable
+pnpm install
+```
+
+## Commands
+
+| Command                        | What it does                                              |
+| ------------------------------- | ---------------------------------------------------------- |
+| `pnpm test`                     | Full test suite, watch mode                                |
+| `pnpm typecheck`                | `tsc --noEmit` across the whole workspace                  |
+| `pnpm lint`                     | `oxlint`                                                    |
+| `pnpm format` / `format:check`  | `oxfmt`                                                     |
+| `pnpm knip`                     | Unused exports/dependencies                                 |
+| `pnpm changeset`                | Add a changeset — see [Versioning](./AGENTS.md#versioning) |
+
+Target one package or one file instead of the whole workspace:
+
+```bash
+pnpm --filter @dunky-dev/core test
+pnpm test packages/core/src/some-file.test.ts
+```
+
+## Storybook
+
+Each UI substrate (React, Vue, ...) is a self-contained package under
+`packages/<substrate>` with its own Storybook — the fastest way to see a
+change actually render. `pnpm dev` delegates to the substrate's package via
+`scripts/sb.js`:
+
+```bash
+pnpm dev             # @dunky-dev/react Storybook, defaults to http://localhost:6006
+pnpm dev vue         # once packages/vue exists
+pnpm build-storybook # static build of the react substrate's Storybook
+```
+
+## Filing an issue
+
+A bug report is only as useful as its reproduction. Use an **SSCCE** — Short,
+Self-Contained, Correct (Compilable) Example:
+
+- **Short** — the smallest snippet that still reproduces it. Strip every
+  prop, wrapper, and state change that isn't load-bearing.
+- **Self-contained** — no missing imports, no "assume you have X set up."
+  Someone should be able to paste it and run it with nothing else.
+- **Correct (compilable)** — it actually runs and actually reproduces the
+  bug. Not what you _think_ is happening — what you pasted and ran.
+
+The [bug report template](.github/ISSUE_TEMPLATE/bug_report.md) asks for
+exactly this. If you can't shrink your repro to an SSCCE, that's often a sign
+the bug isn't where you think it is — shrinking it is frequently how you find
+the real cause yourself.
+
+## Pull requests
+
+### Humans
+
+When using AI to help write changes, read the diff like you wrote it yourself
+before asking someone else to. Cut comments that don't say anything, drop
+anything that drifted from what you're actually fixing, and be able to
+explain any line if asked. A reviewer's time isn't free.
+
+### AI
+
+Before opening it, make sure tests, lint, and typecheck pass, and a changeset is
+included — see `AGENTS.md`'s Workflow section.