docs: add missing AGENTS.md (and README where missing)

AGENTS.md

@@ -0,0 +1,26 @@
+# AGENTS.md
+
+## What this repo is
+
+This repository holds **binary releases only** for `sg`, the Sourcegraph
+developer tool. It contains no application source code. The actual `sg` source
+lives in the Sourcegraph monorepo at
+[`dev/sg`](https://github.com/sourcegraph/sourcegraph/tree/main/dev/sg#readme).
+
+The only tracked file is `README.md`, which records the latest release
+timestamp. Release binaries are distributed via GitHub releases rather than
+committed to the working tree.
+
+## Working in this repo
+
+- There is **no build, test, or lint step** here — there is no code to compile.
+- Do **not** add source code, package manifests, or CI build pipelines; changes
+  to `sg` itself belong in the upstream Sourcegraph monorepo.
+- Releases are produced by upstream automation. The `Latest release:` line in
+  `README.md` is updated as part of that release process.
+
+## Conventions
+
+- Keep changes minimal and scoped to release/documentation metadata.
+- For anything related to `sg`'s behavior, features, or bugs, refer to the
+  upstream project at `github.com/sourcegraph/sourcegraph` (`dev/sg`).