docs(kubernetes): fix agent-sandbox install manifest URL (#11410)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work.
> - The Kubernetes sandbox provider supports the Sandbox CR backend.
> - Its migration guide tells users to install the agent-sandbox
controller.
> - The command names install.yaml, which is not a release asset.
> - This pull request changes the asset name to sandbox.yaml.
> - Users can then download the base controller and CRD manifest.

## Linked Issues or Issue Description

**Issue type**
Incorrect information.

**Where is the issue?**
packages/plugins/sandbox-providers/kubernetes/README.md, under
“Migrating from job to sandbox-cr”.

**What's wrong?**
The installation command uses releases/latest/download/install.yaml. The
upstream agent-sandbox release does not publish that asset.

**Suggested fix**
Use releases/latest/download/sandbox.yaml, the base installation
manifest.

## What Changed

- Replace install.yaml with sandbox.yaml in the controller installation
command.

## Verification

- Checked upstream release v1.0.8. Its assets are sandbox.yaml,
extensions.yaml, and sandbox-with-extensions.yaml.
- Reviewed the one-line documentation diff.
- No Kubernetes installation or local tests were run.

## Risks

- Low risk. This changes one documentation URL.
- The URL follows the latest upstream release, so future upstream asset
changes can affect it.

## Model Used

Original author model use was not reported. Adoption and description
update assisted by OpenAI GPT-6 (Codex), with repository and GitHub tool
access. A more specific runtime model ID is not exposed in this session.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [ ] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [ ] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [ ] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [ ] I have run tests locally and they pass
- [ ] I have added or updated tests where applicable
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [ ] All Paperclip CI gates are green
- [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

Co-authored-by: Andrew Aymeloglu <aaymeloglu@gmail.com>

packages/plugins/sandbox-providers/kubernetes/README.md

@@ -44,7 +44,7 @@ The plugin supports two backend modes, selected via the `backend` config field:
 
 ### Migrating from `job` to `sandbox-cr`
 
-1. Install the agent-sandbox controller: `kubectl apply -f https://github.com/kubernetes-sigs/agent-sandbox/releases/latest/download/install.yaml`
+1. Install the agent-sandbox controller: `kubectl apply -f https://github.com/kubernetes-sigs/agent-sandbox/releases/latest/download/sandbox.yaml`
 2. Update your environment config to set `backend: "sandbox-cr"` (or remove `backend` since `sandbox-cr` is the default)
 3. New leases will use the Sandbox CR backend. Existing leases created with `job` mode continue to use job semantics until they are released.