Add Cursor Cloud dev environment instructions (AGENTS.md) (#7)

Co-authored-by: Cursor Agent <cursoragent@cursor.com>

Author: schacon

AGENTS.md

@@ -37,3 +37,42 @@ reader?** If yes, a human writes it. If the agent is moving, replacing, illustra
 researching, or planning around prose that a human wrote, that's fair game.
 
 When in doubt, treat it as prose and hand it to a human.
+
+## Cursor Cloud specific instructions
+
+This repo is the **Pro Git** book (source in AsciiDoc). "Building" means converting
+`progit.asc` into HTML/EPUB/FB2/MOBI/PDF. There is no runtime backend or database.
+
+### Toolchains
+- **Ruby/Rake pipeline** (this is what CI runs). Ruby 3.2 is installed via apt; Bundler
+  is installed system-wide and gems are installed into `~/.bundle-gems` (configured via a
+  global `bundle config path`). The startup update script runs `bundle install` to refresh
+  gems, so you do not need to reinstall them.
+- **Rust `progit` CLI** in `builder/` — an optional single-command wrapper around the same
+  Asciidoctor toolchain, plus a local book reader. It has no third-party crates. Build with
+  `cargo build --release --manifest-path builder/Cargo.toml` (binary at
+  `builder/target/release/progit`). See `builder/README.md`.
+
+### Build / test / lint (standard commands live in `Rakefile` and `README.asc`)
+- Full build (CI parity): `bundle exec rake book:build`. This also runs the
+  htmlproofer + epubcheck validation step, but **`book:build` intentionally rescues and
+  ignores those check errors** (see the `rescue` in `Rakefile`).
+- Single formats: `bundle exec rake book:build_html` / `build_epub` / `build_fb2` /
+  `build_mobi` / `build_pdf`. The PDF build is the slow one (~1 min).
+- There is no separate lint step; htmlproofer/epubcheck (run via `book:check`) are the de
+  facto linters for the generated HTML/EPUB.
+
+### Non-obvious gotchas
+- **Do not use `bundle exec rake book:ci` for offline verification.** Unlike `book:build`,
+  `book:ci` does *not* ignore htmlproofer/epubcheck errors, and htmlproofer will fail on
+  external links (404s / no network) even when the book itself is fine. Use `book:build`.
+- **MOBI is effectively a no-op.** `book:build_mobi` reports success but does not emit
+  `progit.mobi` — current `asciidoctor-epub3` (see `Gemfile`) no longer produces KF8/MOBI.
+  This is pre-existing repo behavior, not an environment problem.
+- Generated outputs (`progit.html`, `progit.pdf`, `progit.epub`, `progit.fb2.zip`, etc.),
+  `book/contributors.txt`, and `Gemfile.lock` are all git-ignored.
+
+### Local reader (manual/GUI verification)
+- `./builder/target/release/progit serve --port 8080` builds the HTML and serves a
+  multi-page reader at `http://127.0.0.1:8080/` (binds `127.0.0.1` only). `/` is the cover +
+  TOC; `/r/<section>` is a single section with sidebar TOC and prev/next links. Ctrl-C to stop.