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 the engine promises
+labels: bug
+---
+
+## SSCCE
+
+A **Short, Self-Contained, Correct (Compilable) Example** — the smallest
+`setup()`/`machine()` config 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.
+
+```ts
+// paste your minimal repro here
+```
+
+## Expected
+
+What you expected it to do.
+
+## Actual
+
+What it actually did.
+
+## Version
+
+- `@dunky.dev/*` version:

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true
+contact_links:
+  - name: Question or discussion
+    url: https://github.com/dunky-dev/state-machine/discussions
+    about: Not a bug? Start a discussion instead.

.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/state-machine.git
+cd state-machine
+nvm use
+corepack enable
+pnpm install
+```
+
+## Commands
+
+| Command                        | What it does                                               |
+| ------------------------------ | ---------------------------------------------------------- |
+| `pnpm test`                    | Full test suite, watch mode                                |
+| `pnpm test:ci`                 | Full test suite, once                                      |
+| `pnpm typecheck`               | `tsc -b` across the whole workspace                        |
+| `pnpm lint`                    | `oxlint`                                                   |
+| `pnpm format` / `format:check` | `oxfmt`                                                    |
+| `pnpm build`                   | Build every publishable package                            |
+| `pnpm changeset`               | Add a changeset — see [Versioning](./AGENTS.md#versioning) |
+
+Target one package or one file instead of the whole workspace:
+
+## Sandbox
+
+Each sandbox renders the same command-palette machine on a different
+substrate — the fastest way to see a change actually work end to end. See
+[`sandbox/README.md`](./sandbox/README.md) for what each one demonstrates;
+the short version:
+
+```bash
+pnpm -C sandbox/react dev
+pnpm -C sandbox/opentui dev  # terminal — needs Bun
+pnpm -C sandbox/native dev   # Expo  — needs a simulator or device
+```
+
+## 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 machine config that still reproduces it. Strip
+  every state, guard, and action 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 RECONCILE step.