docs(sandbox-providers): add sandbox runtime requirements

State the sandbox runtime environment as a contract with the sandbox
owner. The owner installs node and each used agent CLI on the PATH, and
supplies a POSIX shell, tar, and writable workspace, home, cache, and
temporary locations. Paperclip probes each CLI with command -v before
launch and fails loudly when the CLI is absent and no install command is
configured. An adapter can configure an optional install-and-reprobe
flow. The firm rule states that the runtime never modifies the login
profile and never sources nvm on the exec path.

The document lives with the provider code so it covers every provider,
not one image. The docker image is one reference implementation of this
contract.

Co-authored-by: Paperclip <noreply@paperclip.ing>

packages/plugins/sandbox-providers/SANDBOX-REQUIREMENTS.md

@@ -0,0 +1,70 @@
+# Sandbox Runtime Requirements
+
+This document states the sandbox environment as a contract. The sandbox owner
+must meet this contract. The Paperclip runtime does not build the environment at
+exec time. The environment is a requirement, not a build step.
+
+This document states requirements. It does not state build steps.
+
+## Required on PATH
+
+- `node` must be installed and on the PATH.
+- Each agent CLI that the run uses must be installed and on the PATH. The set of
+  agent CLIs includes `claude`, `codex`, `gemini`, and similar CLIs.
+- The owner installs only the CLIs that the run uses. The owner does not need to
+  install a CLI that no run uses.
+
+## Runtime dependencies
+
+The sandbox execution and synchronization paths need more than `node` and the
+agent CLIs. The owner must also supply these:
+
+- A POSIX shell as `sh`, normally `/bin/sh`. The runtime runs each command with
+  `sh -c <script>`. The runtime uses `bash` only when the adapter sets the shell
+  to `bash`.
+- `tar`. The synchronization path extracts and creates archives with `tar`. A
+  sandbox without `tar` cannot receive or return workspace files.
+- A writable workspace directory. The runtime extracts the workspace archive
+  into this directory.
+- A writable home directory. The agent CLIs write state and credentials under
+  the home directory.
+- A writable cache directory and a writable temporary directory. The runtime and
+  the agent CLIs write intermediate files to these locations.
+
+## Detection contract
+
+Paperclip probes each CLI before launch. Paperclip uses the same detection
+pattern that the runtime Dockerfiles use:
+
+```bash
+command -v <cmd> || exit 1
+```
+
+Paperclip probes each CLI with `command -v <cmd>`. Paperclip fails loudly when
+the CLI is absent and no install command is configured for the CLI.
+
+## Optional CLI installation
+
+An adapter can configure an install command for a CLI. When an install command
+is configured, the runtime obeys this flow:
+
+1. The runtime probes the CLI with `command -v <cmd>`.
+2. If the CLI is already on the PATH, the runtime skips the install.
+3. If the CLI is absent, the runtime runs the configured install command one
+   time.
+4. A failed install is not fatal. The runtime writes a log line and continues.
+   The launch-time probe still reports a missing CLI and fails loudly.
+
+An owner who relies on a configured install command must also supply the network
+access, the filesystem write access, and the package tooling that the install
+command needs. When no install command is configured, the runtime does not
+install the CLI. The owner must supply the CLI on the PATH.
+
+## Firm rule
+
+- The Paperclip runtime never modifies the login profile. The runtime never
+  writes a profile file. The runtime never writes an rc file.
+- The Paperclip runtime never sources `nvm` on the exec path.
+- The sandbox owner supplies a ready PATH. The PATH must resolve `node` and each
+  used agent CLI without any action from the runtime, except for a configured
+  install command.