docs: add repository guidelines

Document repository structure, development commands, testing expectations, and release-only handling for generated lib/index.js.

Co-Authored-By: Codex <codex@openai.com>

AGENTS.md

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+# Repository Guidelines
+
+## Agent-Specific Instructions
+
+Use English for repository-facing content, including documentation, code comments, commit messages, and pull request title/body/comments, unless the task explicitly requires another language.
+
+## Project Structure & Module Organization
+
+This is a TypeScript GitHub Action for publishing GitHub Pages. Source files live
+in `src/`; `src/index.ts` is the entry point and delegates to modules such as
+`get-inputs.ts`, `git-utils.ts`, and `set-tokens.ts`. Tests live in `__tests__/`,
+with static fixtures under `__tests__/fixtures/`. Documentation is in `README.md`
+and `CHANGELOG.md`; screenshots and docs assets are in `images/`. `action.yml`
+defines action inputs and points to generated `lib/index.js`.
+
+## Build, Test, and Development Commands
+
+Use Node `>=20.11.0` and npm `>=10.2.4`.
+
+- `npm ci`: install dependencies from `package-lock.json`.
+- `npm run all`: run format, lint, and Jest tests.
+- `npm run build`: bundle `src/index.ts` into `lib/` with `ncc`.
+- `npm run tsc`: run the TypeScript compiler checks.
+- `npm test`: run Jest with coverage and verbose output.
+- `npm run lint` / `npm run lint:fix`: check or fix ESLint issues.
+- `npm run format:check` / `npm run format`: check or apply Prettier formatting.
+
+When changing `src/`, build locally if needed for verification, but do not commit
+`lib/index.js` in normal changes. Generated `lib/index.js` is placed on `main`
+only during releases.
+
+## Coding Style & Naming Conventions
+
+Write strict TypeScript and keep module boundaries small. Use 2-space
+indentation, semicolons, single quotes, no trailing commas, and a 100-character
+print width, matching `.prettierrc.json`. Test files use `*.test.ts`. Prefer
+names that match action inputs where relevant, for example `publish_branch`.
+
+## Testing Guidelines
+
+Jest with `ts-jest` is the test framework. Add focused tests in `__tests__/` near
+related behavior, and place reusable filesystem samples in `__tests__/fixtures/`.
+Run `npm test` for coverage, and use `npm run all` before opening a pull request
+that touches TypeScript source or tests.
+
+## Commit & Pull Request Guidelines
+
+Recent history follows conventional commit prefixes such as `fix(deps):`,
+`chore(deps):`, `docs:`, and `ci:`. Use a concise imperative subject and include
+the scope when it clarifies the affected area. Pull requests should describe the
+change, link related issues, list verification commands, and include screenshots
+only for documentation image changes. Mention any action input or behavior change
+explicitly so reviewers can assess compatibility.
+
+## Security & Configuration Tips
+
+Do not commit tokens, private keys, or generated secrets. Treat inputs such as
+`deploy_key`, `github_token`, and `personal_token` as sensitive, and keep examples
+limited to placeholder values.