Feature Request: Please add DeepSeek Harness as an Agent harness

**Motivation**

DeepSeek Harness (`dsh`) is an open-source, plugin-oriented agent harness from
DeepSeek AI. It would provide another useful coding and research-oriented
runtime for agents working in Buzz rooms.

At present, DeepSeek Harness is not available in Buzz's **Agent harness**
selector. Users who want to try it must manually determine the installation
method, prerequisites, ACP command, provider configuration, authentication, and
troubleshooting steps.

**Proposed solution**

Add **DeepSeek Harness (`dsh`)** as a supported entry in the existing **Agent
harness** selector used when creating or editing an agent.

The entry should:

- Display DeepSeek Harness as a selectable harness.
- Provide an installation and setup guide.
- Detect whether the required `dsh` components and prerequisites are installed.
- Document and launch the supported DeepSeek Harness ACP command.
- Provide guidance for configuring the required model provider credentials.
- Show whether the harness is installed, configured, and ready.
- Provide actionable errors for missing prerequisites, authentication, and
launch failures.
- Use Buzz's standard `buzz-acp` / Agent Client Protocol integration.
- Allow DeepSeek Harness to be selected for an individual Buzz agent.
- Clearly identify any developer-preview or compatibility limitations.

DeepSeek Harness appears to provide an automation-oriented ACP server over
JSON-RPC stdio. Please validate the correct supported launch command and
configuration for Buzz, and coordinate with the upstream maintainers if any
compatibility changes are needed.

**Alternatives considered**

1. **Custom harness**

Users could configure DeepSeek Harness manually through Buzz's Custom harness
option. This is less discoverable and requires users to find the correct
`dsh` command, arguments, environment variables, provider settings, and ACP
requirements themselves.

2. **Running DeepSeek Harness separately**

Users could run `dsh` outside Buzz, but that prevents it from being selected
and managed as an agent inside Buzz rooms alongside the other available
harnesses.

3. **A user-maintained ACP adapter**

Users could build their own adapter, but this would duplicate integration
work and create compatibility and support problems as Buzz or DeepSeek
Harness changes.

**Additional context**

Upstream project:

- Repository: https://github.com/deepseek-ai/deepseek-harness
- Project name: DeepSeek Harness
- Common runtime/package name: `dsh`

The DeepSeek Harness repository documents an automation-oriented Agent Client
Protocol server over JSON-RPC stdio. Its ACP documentation states that stdout
is reserved for newline-delimited ACP JSON-RPC messages, which appears relevant
to Buzz's ACP runtime architecture.

The requested UI location is the existing **Agent harness** selector shown when
editing an agent. This request is for a harness entry in that selector, not for
a change to Buzz's agent hierarchy or dashboard.

Could the maintainers please document the supported `dsh` ACP launch command,
required prerequisites, provider configuration, supported operating systems, and
any developer-preview limitations if this integration is added?