docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to WebHooker
+
+Thanks for your interest in contributing! WebHooker is a Nuxt 4 (Nitro) app that runs as a
+Cloudflare Worker, receiving GitHub/Gitea webhooks and dispatching them to Discord and Telegram.
+
+## Development Setup
+
+[Bun](https://bun.sh) is the only supported package manager — never use `npm`/`npx`.
+
+```bash
+git clone https://github.com/ReCloudStudio/WebHooker.git
+cd WebHooker
+bun install
+cp .env.example .dev.vars   # Fill in secrets for local dev (wrangler reads this)
+bun run dev                 # Start Nuxt dev server (HMR + Nitro)
+```
+
+## Scripts
+
+| Command                | Description                              |
+| ---------------------- | ---------------------------------------- |
+| `bun run dev`          | Start Nuxt dev server                    |
+| `bun run build`        | Production build (cloudflare preset)     |
+| `bunx wrangler dev`    | Preview a built worker (Miniflare)       |
+| `bun run typecheck`    | TypeScript type checking                 |
+| `bun run lint`         | ESLint (TypeScript)                      |
+| `bun run lint:md`      | Markdownlint                             |
+| `bun test`             | Run the unit-test suite                  |
+| `bun run format`       | Format all files with Prettier           |
+| `bun run format:check` | Check Prettier formatting                |
+| `bun run docs:dev`     | Start the VitePress docs dev server      |
+| `bun run db:migrate`   | Apply D1 migrations locally              |
+
+## Code Style
+
+- TypeScript in strict mode
+- Double quotes, semicolons required, trailing commas, 100-char print width (Prettier)
+- ESLint with `@typescript-eslint` recommended rules; Markdownlint for Markdown
+- User-facing strings are bilingual (en + zh) in `app/composables/useI18n.ts`
+
+## Testing
+
+```bash
+bun test
+```
+
+The suite includes per-module unit tests, provider fixtures (`tests/fixtures/`), formatter
+snapshots (`tests/__snapshots__/`), and platform contract tests. CI runs
+`bun install --frozen-lockfile`, `bun test`, and `bun run lint`.
+
+## Commit Guidelines
+
+- All commits must be GPG-signed
+- Write concise, accurate messages in English using conventional-commit style
+  (e.g. `feat(filters): ...`)
+- Keep changes focused and atomic
+
+## Documentation
+
+Every functional change must keep its docs in sync: `AGENTS.md`, `README.md` / `README.zh.md`,
+the VitePress docs (`docs/` and `docs/zh/` mirrors), and `config.example.yaml` / `.env.example`.
+Code and docs must not drift.
+
+## Pull Requests
+
+- Run `bun run typecheck && bun run lint && bun test` before submitting
+- Include type annotations for all function returns
+- Update documentation for any feature or behavior change
+
+For the full guide — including the project structure and how to add a new event formatter — see
+[`docs/contributing.md`](docs/contributing.md).