fix: bun_install adds node_modules to each directory

docs/bun_install.md

@@ -0,0 +1,90 @@
+# `bun_install`
+
+`bun_install` is a Bzlmod module extension for creating an external repository
+that contains a Bun-generated `node_modules/` tree.
+
+Unlike the build rules in [rules.md](rules.md), `bun_install` is not loaded from
+`@rules_bun//bun:defs.bzl`. It is loaded from
+`@rules_bun//bun:extensions.bzl`, so it is documented separately.
+
+## What it does
+
+`bun_install`:
+
+- runs `bun install --frozen-lockfile`
+- uses your checked-in `package.json` and `bun.lock` or `bun.lockb`
+- creates an external Bazel repository exposing `:node_modules`
+- keeps dependency installation under Bun rather than npm
+
+The generated repository can then be passed to rules such as `bun_script`,
+`bun_binary`, `bun_bundle`, and `bun_test`.
+
+## Usage
+
+```starlark
+bun_install_ext = use_extension("@rules_bun//bun:extensions.bzl", "bun_install")
+
+bun_install_ext.install(
+    name = "bun_deps",
+    package_json = "//:package.json",
+    bun_lockfile = "//:bun.lock",
+)
+
+use_repo(bun_install_ext, "bun_deps")
+```
+
+Then reference the installed dependencies from build targets:
+
+```starlark
+load("@rules_bun//bun:defs.bzl", "bun_script")
+
+bun_script(
+    name = "web_dev",
+    script = "dev",
+    package_json = "package.json",
+    node_modules = "@bun_deps//:node_modules",
+)
+```
+
+## `install(...)` attributes
+
+### `name`
+
+Repository name to create.
+
+This becomes the external repository you reference later, for example
+`@bun_deps//:node_modules`.
+
+### `package_json`
+
+Label string pointing to the source `package.json` file.
+
+Example:
+
+```starlark
+package_json = "//:package.json"
+```
+
+### `bun_lockfile`
+
+Label string pointing to the Bun lockfile.
+
+Supported lockfile names are:
+
+- `bun.lock`
+- `bun.lockb`
+
+Example:
+
+```starlark
+bun_lockfile = "//:bun.lock"
+```
+
+## Notes
+
+- `bun_install` runs Bun, not npm.
+- The repository name is arbitrary. `bun_deps` is only an example.
+- The generated repository exposes a standard `node_modules/` tree because that
+  is the dependency layout Bun installs.
+- `--frozen-lockfile` is used, so the lockfile must already be in sync with
+  `package.json`.