docs: add repository agent guide (#888)

AGENTS.md

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+# AGENTS.md
+
+## Repository purpose
+
+Papers We Love is a curated directory of academic computer-science papers.
+Most changes add or correct entries in topic-level `README.md` files; a smaller
+set of papers is hosted in the repository. This is not an application project:
+there is no package manager, build, or unit-test suite.
+
+## Layout
+
+- The root `README.md` explains the collection and points contributors to the
+  project policies.
+- Each topic directory owns a `README.md` index. Some topics have nested
+  subtopics; keep an entry with the most specific appropriate topic.
+- Hosted papers live beside the topic index. `scripts/download.sh` retrieves
+  linked PDFs but is not part of a normal documentation-change check.
+- `.github/CONTRIBUTING.md` is the source of truth for contribution policy;
+  `.github/PULL_REQUEST_TEMPLATE.md` lists the information expected in a PR.
+
+## Editing paper indexes
+
+- Preserve the target README's existing headings, ordering, Markdown style,
+  and level of description. Do not reformat unrelated entries.
+- Add an academic paper, not a book, blog post, or general reference. Include
+  a short explanation of its importance when adding a paper, as required by
+  the contribution guide.
+- Include the publication year with a new listing when it is known.
+- Prefer a stable, direct source link and verify that the link reaches the
+  intended paper before submitting it.
+- Use `:scroll:` for a repository-hosted paper. A hosted paper must also have
+  its source URL recorded in that topic's README.
+- Respect copyright: only add a paper file when its license permits digital
+  redistribution. Otherwise, link to the canonical external source instead.
+- Name new topic directories in lowercase with underscores; name paper files
+  in lowercase with dashes and use the full title where practical.
+
+## Scripts and validation
+
+- Run `bash -n scripts/download.sh` after changing the downloader.
+- The CI workflow checks Markdown links with Lychee. For edited entries,
+  manually validate new or changed URLs; run a local Lychee check when it is
+  available rather than treating a successful Markdown parse as link
+  validation.
+- Inspect `git diff --check` and the focused diff before handoff. Confirm that
+  each hosted-paper link resolves to the file committed alongside its README.
+- Do not run `./scripts/download.sh` as a routine check: it traverses the
+  collection, downloads network resources, and can create many PDF files.
+  Use it only when deliberately refreshing a clearly scoped topic.
+
+## Change hygiene
+
+- Keep each contribution focused on a paper, topic, link repair, or script
+  maintenance task.
+- Do not modify or remove existing PDFs merely because a downloader finds a
+  different copy.
+- Follow the Code of Conduct and keep PRs ready to squash into one commit, per
+  the contribution guide.